I’ve got upgraded on every GHA stay in the recent years. Especially the Kempinskis were generous (2x upgrade because of Titanium plus 1x upgrade because of FHR gave 4x upgrade or better).
I’ve also had an issue about a missing stay (booked via GHA app) and the GHA processes don’t work at all. They need long time getting nothing done.
So it’s a good program for getting upgrades, but not a reliable one.