That would depend on whether or not it is a new project written while this jam/compo is running.
Here's the relevant rule:
https://gbdev.io/gbcompo21/#rules
17. 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)
Examples of things that are not allowed:
- A project you’ve been working on for months.
- An update/patch for a game you already released publicly.