Arguably the best workflow to add GPS data (from a GPX track) to pictures is to assign it in GC before importing to Photos
If the picture is already in the macOS Photos library, I can export it, delete it in Photos (also in Recently Deleted), then tag it in GC and reimport to Photos, that works.
I also tried to use the "edit with" feature of Photos to edit a picture in GC (using GC12.1), but there it seems I can't assign the GPS location (can you do that only in a Browser?)
Anyway, I tried also to "edit with GC" from Photos, which places the picture in a random subfolder (UUID) of the "name-of-the-library".photoslibrary/private/com.apple.photos/ExternalEditSession/ folder, and then opening a GC browser there while GC has the picture open, and assign the GPS location there. Although GC confirms writing the GPS location, it doesn't show back in Photos.
Do you know of a better way to tag pictures that are already in macOS Photos?
Kind regards, narium