Hi Alwin, great questions!
You can create your repo anytime, even right at the end with a single commit. There’s no requirement to show progress publicly throughout the jam. Use whatever workflow works best for you or your team.
Your project doesn’t need to be “open source,” just “source available,” so others can potentially build and learn from it (or help confirm the code was written during the jam period). You’re encouraged to license it however you like and keep developing it afterward, even for a commercial release. There’s no expectation to share any proprietary or purchased assets; placeholders or omissions are totally fine, and lots of folks do that.
The “source available” idea goes back to the early days of GitHub. Before Game Off started around 2012, I used to share zip files of my game jam projects with colleagues and upload them to jam sites. It struck me how few examples of game jam code were out there and how hard they were to find. GitHub was full of web frameworks and libraries back then, but it was kind of a graveyard for finished games.
The original model of participation, before itch.io even existed, was to fork the Game Off repo (for example, github.com/github/game-off-2012) and build your game there. I even built an overly elaborate voting system for judges using the GitHub API. We started using itch.io in 2017, which made running the jam and submitting and playing entries a whole lot easier!
I started Game Off to make game development a little more open, discoverable, and inspiring for everyone, especially for my kids one day who might want to learn about game development too. (I got into software by hacking games as a kid.)
It’s still a side project and one of the highlights of my year, more than a decade later. Believe it or not, this is the short version of that origin story, but hopefully it gives some helpful context! :D
Cheers,
Lee