i don’t think there is any default Game Boy UI. i’m not sure what you mean by that. please refer to the FAQ:
What can I work on before the jam?
You are allowed to set up your tools and your game engine. Tools include: palettes, fonts, tracker instruments and code libraries.
if you’re asking about fonts, that is also cleared up in the FAQ:
Can I use a public domain font?
Yes. You do not have to make your own font for this jam.
