So, I didn't realize this a few days ago when I started this process, but today I was reading about the GPL and came upon this fact:
I downloaded just the binary from the net. If I distribute copies, do I have to get the source and distribute that too?
Yes. The general rule is, if you distribute binaries, you must distribute the complete corresponding source code too. The exception for the case where you received a written offer for source code is quite limited.
https://www.gnu.org/licenses/gpl-faq.html#UnchangedJustBinary
This means that distributing the .hex files as we are may actually violate the terms of the GPL license itself. We have to distribute the source code in the repo for these games as well.
I'm not sure if linking to the source code on a different server is enough. Perhaps we should find this out.