The source and assets you're able to distribute have to be in a public GitHub repository, but it doesn't need to be "open source".
You're under no obligation to choose an open source license. And if you don't use any license, the default copyright laws apply, meaning that you retain all rights to your source code and no one may reproduce, distribute, or create derivative works from your work. See https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-f...,
Nobody will be judging your code if that helps. Hope this makes it a little easier to consider participating :)