Just download the repository and drop the gamejolt_api folder into your addons folder. Open the add node dialog and locate GameJoltAPI under the HTTPRequest node. A demo project showing some of the functions is next to the plugin folder.
Before doing any calls to the API you must authenticate a user first so the system knows what user to work with. A method to do it is provided:
auth_user(token, username)
token - your gamejolt user token(NOT your password) username - your username