As long as it’s done in good faith (that is to say, you didn’t write a bunch of code specifically for the jam game with intent to reuse it when the jam starts) you’re welcome to reuse any packages/systems you’ve developed in the past. Especially when it comes to things like Unity and Game Maker, people use a lot of premade packages to handle a lot of different things and save time, and that’s totally fine. I’d rather see everyone make the coolest things they can than see them stuck doing very basic groundwork