How to convert time offset value to GMTformat in Java

I get from remote API such kind body:

    {
       "timezone": "18000",
       "id": "1512569",
       "name": "Tashkent"
    }

How can I convert timezone field to GMT format? For example in my case:

timezon:18000  = GMT 5

You can convert the given number of seconds into a ZoneOffset instance and then format it the desired string e.g.

import java.time.ZoneOffset;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeFormatterBuilder;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        DateTimeFormatter formatter = new DateTimeFormatterBuilder()
                                        .appendLiteral("GMT")
                                        .appendOffset("+H:mm", "Z")
                                        .toFormatter(Locale.ROOT);

        ZoneOffset offset = ZoneOffset.ofTotalSeconds(18000);
        String formattedOffset = formatter.format(offset);
        System.out.println(offset);
        System.out.println(formattedOffset);
    }
}

Output:

+05:00
GMT+5

Learn more about the the modern date-time API from Trail: Date Time.