The Google Cloud Platform exams do have a lot of similarities to the AWS exams, but in our opinion, for most people, the GCP Professional Cloud Architect exam will be more difficult than the AWS Certified Solutions Architect Professional exam.
One difference is that Google expects every Solutions Architect to come from a dev background. Now this might not be an issue for most people, but some would be quite thrown off by a question asking them to debug some Python code that has nothing to do with GCP.
But this leads into the key difference, which is that the GCP certification revolves entirely around professionally performing the Cloud Architect role, rather than around the platform itself. There is significant value in this because it can better-represent real-world ability. But this can also make it much harder because the scope is almost limitless. Many exam questions have nothing to do with Google.
Furthermore, although AWS and Google ask almost only scenario questions on their Pro exams, we felt that the way that Google asked their questions required to bridge even wider gaps in the question and its responses, to determine which were the right answers. What we mean by this is that a question might specify information A and a response might talk about C and G, but you have to imagine all the services, requirements, and business motivations filling in B, D, E, and F, in order to determine whether that option is reasonable. AWS does this, too (and it's one of the things mentioned in the Cert Prep Guide), but Google did this even more, in our opinion.