GitHub's Game Off event invites participants to creatively modify existing games or game jam entries hosted on GitHub, with a focus on the theme "the game has changed." Participants can work individually or in teams to enhance games by altering elements like sprites, soundtracks, or levels, and are encouraged to utilize open-source resources. The only requirement is that the game must be web-based. Interested individuals can explore past Game Off winners and other game jam entries on GitHub to find inspiration. Participants need a GitHub account to fork a game repository and submit their modified game by April 13th. GitHub provides guidance and support for those new to version control, and the event encourages engagement through the Twitter hashtag #ggo15.