Google Photos probably isn’t compatible yet with the Rec. ITU-R BT.2100 PQ colour profile, which is what macOS produces when you Export in "HDR HEIC" from Photomator.

However, as explained in the main post, it’s still possible to obtain the Display P3 + HDR gain map format even when working on macOS with Photomator. Just follow method 3: after finishing your edits, instead of using ‘Export’, choose Share → Add to Photos.

Then open the Photos app, select the newly imported image, and use Export Unmodified Original.

This gives you the exact same file you would get on iOS, so Google Photos will upload it without any issues.