Convert each list to map and merge it (I use Lombok to not write boilerplate code):

@Data
@NoArgsConstructor
@AllArgsConstructor
class Server {
    private String name;
    private String attribute1;
    private String attribute2;
}

public class ServerMain {

    public static void main(String[] args) {

        List<Server> servers1 = Arrays.asList(
                new Server("name1", "attr1.1", null),
                new Server("name2", "attr1.2", null));

        List<Server> servers2 = Arrays.asList(
                new Server("name1", null, "attr2.1"),
                new Server("name2", null, "attr2.2"));

        Map<String, Server> serverMap1 = servers1.stream().collect(Collectors.toMap(Server::getName, Function.identity()));
        Map<String, Server> serverMap2 = servers2.stream().collect(Collectors.toMap(Server::getName, Function.identity()));

        serverMap1.keySet().forEach(key -> serverMap1.merge(key,
                serverMap2.get(key),
                (server1, server2) -> {
                    server1.setAttribute2(server2.getAttribute2());
                    return server1;
                }));


        System.out.println(serverMap1);
    }
}

In Java 8:

List<String> newList = Stream.concat(listOne.stream(), listTwo.stream())
                             .collect(Collectors.toList());

Java 16+:

List<String> newList = Stream.concat(listOne.stream(), listTwo.stream()).toList();

You will need to make use of Google's gson library to parse your json Strings and also create Additional Classes to hold the data for the same. Save your json String in Files to read the content and convert them to appropriate Objects.

The JSON Strings you have mentioned have error since you are using (“) at many places. Replace them with (") at all locations. This change is very important because if you don't do it you might get Exceptions or undesirable outputs.

Store your Json strings as follows:

List1.txt

{
"One": [
    "G13DA"
],
"Two": [
    "G13AA",
    "G13AD"
]
}

List2.txt

[
{
    "subgroupCode": "G13AA",
    "productGroup": [
        {
            "productCode": "G1A",
            "productDescription": "Two 1"
        },
        {
            "productCode": "G9B",
            "productDescription": "Two 2"
        }
    ]
},
{
    "subgroupCode": "G13AD",
    "productGroup": [
        {
            "productCode": "G4A",
            "productDescription": "Two 3"
        },
        {
            "productCode": "G9C",
            "productDescription": "Two 4"
        }
    ]
},
{
    "subgroupCode": "G13DA",
    "productGroup": [
        {
            "productCode": "G5M",
            "productDescription": "One 1"
        },
        {
            "productCode": "G9M",
            "productDescription": "One 2"
        }
    ]
}
]

You will need to change the file paths of these two while Running the Program

Add the following classes

JuniorArea.class

public class JuniorArea {
    private String juniorAreaDescription;
    private List<MerchandisingGroup> mechandisingGroup;

    public String getJuniorAreaDescription() {
        return juniorAreaDescription;
    }
    public void setJuniorAreaDescription(String juniorAreaDescription) {
        this.juniorAreaDescription = juniorAreaDescription;
    }
    public List<MerchandisingGroup> getMechandisingGroup() {
        return mechandisingGroup;
    }
    public void setMechandisingGroup(List<MerchandisingGroup> mechandisingGroup) {
        this.mechandisingGroup = mechandisingGroup;
    }

    @Override
    public String toString() {
        return "JuniorArea [juniorAreaDescription=" + juniorAreaDescription + ", mechandisingGroup=" + mechandisingGroup
                + "]";
    }
} 

JuniorAreaCollection.class

public class JuniorAreaCollection {
    private List<JuniorArea> juniorArea;

    public List<JuniorArea> getJuniorArea() {
        return juniorArea;
    }

    public void setJuniorArea(List<JuniorArea> juniorArea) {
        this.juniorArea = juniorArea;
    }

    @Override
    public String toString() {
        return "JuniorAreaCollection [juniorArea=" + juniorArea + "]";
    }
}

MerchandisingGroup.class

public class MerchandisingGroup {
    private String productCode;
    private String productDescription;

    public String getProductCode() {
        return productCode;
    }
    public void setProductCode(String productCode) {
        this.productCode = productCode;
    }
    public String getProductDescription() {
        return productDescription;
    }
    public void setProductDescription(String productDescription) {
        this.productDescription = productDescription;
    }

    @Override
    public String toString() {
        return "MechandisingGroup [productCode=" + productCode + ", productDescription=" + productDescription + "]";
    }
}

Product.class

public class Product {
    private String subgroupCode;
    private List<ProductGroup> productGroup;

    public String getSubgroupCode() {
        return subgroupCode;
    }
    public void setSubgroupCode(String subgroupCode) {
        this.subgroupCode = subgroupCode;
    }
    public List<ProductGroup> getProductGroup() {
        return productGroup;
    }
    public void setProductGroup(List<ProductGroup> productGroup) {
        this.productGroup = productGroup;
    }

    @Override
    public String toString() {
        return "Product [subgroupCode=" + subgroupCode + ", productGroup=" + productGroup + "]";
    }
}

ProductGroup.class

public class ProductGroup {
    private String productCode;
    private String productDescription;

    public String getProductCode() {
        return productCode;
    }
    public void setProductCode(String productCode) {
        this.productCode = productCode;
    }
    public String getProductDescription() {
        return productDescription;
    }
    public void setProductDescription(String productDescription) {
        this.productDescription = productDescription;
    }

    @Override
    public String toString() {
        return "ProductGroup [productCode=" + productCode + ", productDescription=" + productDescription + "]";
    }
}

Finally run the follwing Main Class code, to check the output

public class MainRunner {
    // Use Path according to your file system
    private static String List1Path = "/List1.txt";

    // Use Path according to your file system
    private static String List2Path = "/List2.txt";

    public static void main(String...arg) throws UnsupportedEncodingException, IOException {
        Gson gson = new GsonBuilder().setPrettyPrinting().create();

        String List1Content = new String(Files.readAllBytes(Paths.get(List1Path)), "UTF-8");

        Map<String, List<String>> map = new HashMap<>();

        map = gson.fromJson(List1Content, new TypeToken<Map<String, List<String>>>() {}.getType());

        System.out.println(map);

        String List2Content = new String(Files.readAllBytes(Paths.get(List2Path)), "UTF-8");

        List<Product> products = gson.fromJson(List2Content, new TypeToken<List<Product>>() {}.getType());

        System.out.println(products);

        List<JuniorArea> juniorAreas = new ArrayList<>();

        for (String s : map.keySet()) {
            JuniorArea juniorArea = new JuniorArea();

            juniorArea.setJuniorAreaDescription(s);

            List<MerchandisingGroup> merchandisingGroups = new ArrayList<>();

            List<String> subgroupCodes = map.get(s);

            for (String subgroupCode : subgroupCodes) {

                for (Product product : products) {

                    if (product.getSubgroupCode().equals(subgroupCode)) {
                        List<ProductGroup> productGroups = product.getProductGroup();

                        for (ProductGroup productGroup : productGroups) {
                            MerchandisingGroup merchandisingGroup = new MerchandisingGroup();

                            merchandisingGroup.setProductCode(productGroup.getProductCode());
                            merchandisingGroup.setProductDescription(productGroup.getProductDescription());

                            merchandisingGroups.add(merchandisingGroup);
                        }
                    }

                }

            }

            juniorArea.setMechandisingGroup(merchandisingGroups);

            juniorAreas.add(juniorArea);
        }

        JuniorAreaCollection collection = new JuniorAreaCollection();

        collection.setJuniorArea(juniorAreas);

        System.out.println(gson.toJson(collection));
    }
}

Output should be printed on your console

{
  "juniorArea": [
    {
      "juniorAreaDescription": "One",
      "mechandisingGroup": [
        {
          "productCode": "G5M",
          "productDescription": "One 1"
        },
        {
          "productCode": "G9M",
          "productDescription": "One 2"
        }
      ]
    },
    {
      "juniorAreaDescription": "Two",
      "mechandisingGroup": [
        {
          "productCode": "G1A",
          "productDescription": "Two 1"
        },
        {
          "productCode": "G9B",
          "productDescription": "Two 2"
        },
        {
          "productCode": "G4A",
          "productDescription": "Two 3"
        },
        {
          "productCode": "G9C",
          "productDescription": "Two 4"
        }
      ]
    }
  ]
}

Assuming that you want to retain non zero marks and non null status elements, you can not directly use distinct. You can use Collectors.toMap as below,

List<User> userList3 = Stream.concat(source1.stream(), source2.stream())
                .collect(Collectors.toMap(e -> e.firstName + e.lastName, e -> e,
                        (User u1, User u2) -> {
                            if (u1.marks == 0)
                                u1.marks = u2.marks;
                            if (u1.status == null)
                                u1.status = u2.status;
                            return u1;
                        })).values().stream().collect(Collectors.toList());

Note that you may need to reorder elements.

If you are using Jackson (one of the most popular JSON libraries), you can simply achieve this as follows:

Code Snippet

ObjectMapper mepper = new ObjectMapper();
List<Map<String, Object>> listAs400 = mepper.readValue(jsonStrAs400, List.class);
List<Map<String, Object>> listSqlServer = mepper.readValue(jsonStrSqlServer, List.class);
listAs400.forEach(e1 -> {
    listSqlServer.forEach(e2 -> {
        if (e1.get("truckNumber").toString().equals(e2.get("truckNumber").toString())) {
            e2.forEach((k, v) -> {
                e1.put(k, v);
            });
        }
    });
});
System.out.println(mapper.writeValueAsString(listAs400));

Console Output

[ 
  { 
    "assetId":308,
    "type":"OV",
    "truckNumber":"L122",
    "longitude":0,
    "latitude":0,
    "id":308,
    "trailerGroup":"C",
    "loaded":false,
    "dedicated":false,
    "intermodal":false,
    "sealed":false,
    "companyOwned":true,
    "onSite":false,
    "customerId":"KTPH",
    "modified":{ 
      "when":1546498401156
    },
    "created":{ 
      "when":1546498401156
    }
  },
  { 
    "assetId":309,
    "type":"PO",
    "truckNumber":"SQ46",
    "longitude":0,
    "latitude":0,
    "id":309,
    "trailerGroup":"C",
    "loaded":false,
    "dedicated":false,
    "intermodal":false,
    "sealed":false,
    "companyOwned":true,
    "onSite":true,
    "customerId":"KTPH",
    "modified":{ 
      "when":1546498401156
    },
    "created":{ 
      "when":1546498401156
    }
  }
]