Great question! Till now I hadn’t noticed what @ennoborg did: branches on origin (gramps-project/gramps) created after forking aren’t created in the fork. Or put another way, I didn’t notice that forking automatically created existing branches in the forked repo? Will have to read more about that.
If I understand Nick’s response he’s saying that you have to track the new maintenance/gramps52 branch via your fork on GitHub. I did that via command line and it worked fine, and if you want to do it via GitHub web, go to your fork > Branches, and then “New Branch” and set the parameters like this and then you will have a 5.2 branch which tracks a branch of the same name on origin:
Learned something new today 