You can currently use Photon custom auth and callbacks with GameSparks, I tried this months ago and it works fine. These links will help you get going:
Thanks - this helps a ton! I'm almost done, I just ran into 1 block:
I am providing an authentication token to prove they are currently authorized: how can I validate an AuthToken received from an AuthenticationResponse to prove they are already logged in in some form?
Seems that this guy has a similar question (he wants to know how to validate the authToken):
Any update on validate the authToken?
One way I've managed to do this is by using the undocumented Spark.systemCollection("player") then find my player document by id then check the authToken I got via photon authentication web hook.
Thank you very much for great tip
For example, I could login to Photon via GameSparks. From there, could capture Photon callbacks and such integrated into your SDK.
Would be super convenient :) Although I recently discovered you guys support realtime, too, so I'm not sure if this would conflict in your business model ~