To write this article I am using this web site.

Open Gedit, and while Gedit window is active go to Edit -> Preferences:

Go to plugins and check external tools:

Now open Manage External Tools:

Click add:

Enter a name (Format JSON), paste this text:

#! /usr/bin/env python

import json
import sys

j = json.load(sys.stdin)
print json.dumps(j, sort_keys=True, indent=2)

Set Input to Current document and set Output to Replace current document: