I was able to successfully download json in my gradle dependencies:

implementation 'org.json:json:20171018'

If you download the JSON jar specified, and list its contents (e.g. with jar tf), it does not contain the org.json.simple package.

So the problem is simply that you need another jar.

EDIT:

I don't know if this is the intent, but an educated guess: if you add this dependency to build.gradle:

compile 'com.googlecode.json-simple:json-simple:1.1.1'

and these imports:

import org.json.simple.parser.*;
// import org.json.simple.*;
import org.json.*;

then the example compiles (for me).

If you are using LibGdx, it does not come with a Gradle setup, making multi-project builds streamlined.

You need to add implementation 'org.json:json:<version>' to the dependencies block of your top-level build.gradle.

To find the latest version, see http://mvnrepository.com/artifact/org.json/json/

For example, using the latest version at the time of writing:

dependencies {
   implementation 'org.json:json:20200518'
}

Your Gradle script is fine but when you run the JAR, you have not included the org.json jar on the classpath.

If you don't want to have to do this, you can create what is called a fat jar which bundles the org.json JAR inside your JAR. Such a process is described here.

There are many other ways to do this (see here and here)so you should research the different options but at the end of the day the org.json JAR needs to be present when you compile, but also when you run the program. At the minute you are only fulfilling the first of those requirements.