Question_Asker Well, I am not a lawyer. But in general the obligation of the GPL is that you must provide the source code upon request from your licensee (which is someone you have sold your GPL-containing product to). That means you must provide it, not point to some upstream source. But there are many ways to fulfill the obligations of the GPL. Technically you don't even have to put the code on an online repository. You can mail it to people on CD if you want lol.
So the simplest way to do this, is to maintain a mirror of the code on a git forge (GitHub, self-hosted Foregjo, GitLab, etc), and then when or if people ask for the code, point them at that link. Or just put the link in a manual. Which is exactly what Volla does.
Edit: and to be honest, maybe upstream source is enough, if your source is entirely upstream? I dunno. Again, not a lawyer.