is it okay to download and use a font from, say, the BitFontMaker2Gallery (which is a great font site for pixel-art games btw) or do we need to make all the typefaces ourselves in the timespan of the jam?

This is a question our team had as well!

I'm no admin, but the main page says:

• 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.

Most jams I've seen allow fonts to be brought in. It seems like the GBJam folks consider fonts to be in the same category as tools.

Yes, fonts are fine :~)

Okay good good just thought I'd make sure, thanks everyone!