I want to use Azure AD to authenticate users to GCP/GKE, is this natively possible (i.e. without using some external service to mimic the Azure AD structure in Google Cloud IAM)
You can combine three Google Cloud systems to do this:
With these together, you can have managed accounts synced from Active Directory, that use your Active Directory to log in. There is a great write up of this set-up on the gcp blog that goes into some more detail.