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).

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

implementation 'org.json:json:20171018'

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'
}

"compile" configuration is no longer used in Gradle (since Gradle 7.0)

Try replacing

compile 'org.json:json:20210307'

with

implementation("org.json:json:20210307")

See this topic for more info.