So Making an actual Gameboy game if you dont already know is going to way more difficult then just using something like Unity or game maker. There are less resources in terms of documentation and help and it is much more complicated then modern engines.

Ask yourself what it is you want to achieve in the jam - is it to make a cool game? if so, then do that in the easiest, most empowering way and make something really unique

If it is to learn how to make an actual game boy game then great! But you probably wont be able to do that AND make a good game in all likely hood and you would probably do better taking longer then a jam to learn actual hardware.

I hope this helps!

Going "full retro" is very nice. Seeing your game played on an actual Game Boy is awesome. It's not a walk in the park, but it's not super hard either, depending on your experience. There are lots of resources here: http://avivace.github.io/awesome-gbdev

I make Game Boy games in assembly myself (although I haven't finished any yet... will try to do this jam though), but if you want to do it quickly (and not spend the entire month of August learning stuff) I'm going to go ahead and suggest checking out the ZGB engine. You code in C, and it has stuff to help you. There was a ZGB jam recently too.

It does use GBDK (Game Boy Development Kit) as a framework, which is not super great, but it helps you a lot along the way.