Google Cloud has exactly the same thing as AssumeRole, it's called Impersonate.

See https://cloud.google.com/docs/authentication/use-service-account-impersonation

Same as with AWS -- you still need to authenticate (using credentials or SSO or whatever), otherwise cloud doesn't know who you are, and then:

1. If your identity has permissions to AssumeRole/Impersonate, and
2. If that Role (AWS) / ServiceAccount (GCP) allows to be impersonated by you

then your identity can act as other assumed/impersonated identity.