If you’re previous/existing work falls under the category of game engine (sounds like it may) then that is ok to re-use as there is some exception for that in the rule requiring work to be new and original (see first item in list below).
Work outside that scope such as levels, etc would be different. Assets can be reused but should be noted if so (would fall under the same category as asset packs).
https://gbdev.io/gbcompo25.html#hardware-criteria
‘#19’
Your work must be new and original. You cannot enter with something you were already working on before the start of the jam. The majority of work should be done during the jam. If in doubt, please ask us before proceeding.
Examples of things that are allowed:
- A brand new game using your own (or any) existing game engine.
- A remake of your (or someone else’s) game for a different platform, but with new code and assets.
- A brand new game you barely started (e.g. you only made a title screen or a mockup of the gameplay).
- Using templates, tutorial code as starting point (when they’re license allow to do so, e.g. gb-boilerplate) ..(continues, see link for full details)..