Everything made FOR a game jam game should be made during the duration if the development time, Though people are usually allowed to use their old sprites and snippits of code from older projects (Small things like a scorekeeper or a common system seen in almost every game). You usually cannot make anything specifically for the Jam, before the developing time starts.
Edit: heres a bit of the rules part of this game jam specifically:
- You may use pre-made, publicly available, and purchased assets, as well as GDevelop's built-in asset library, or other assets you have the legal rights to use. Just remember to credit properly.
- You can use pre-existing code, extensions, functions. But do not write code specifically for this jam, ahead of time.