I'm not sure about GCP, but Azure does offer role-based access[1] which gives you similar resource authentication magic to what IAM provides. The definition formats[2] even look fairly close to their IAM equivalents.
It's used in combination with Azure Active Directory, so the modality isn't 1:1 with AWS. But Managed Identities[3] is a feature that's rolling out across Azure which simplifies the model a bit, since it negates the need to create service principles in AAD beforehand.