GQL you would only care about if someone removed a field not if someone added a field. How would adding a field change existing GQL calls return? Doesn't make sense.
Also, Its about 1 line to set up CI to extract all GQL queries from a typescript project and do static analysis to check against the schema.
But again you only care if someone deletes a field, and even if you have to delete it, at least the GQL schema has built in annotations for deprecation, something not in REST.