Two things I would recommend:
Subscription to A Cloud Guru - they have a lot of content on GCP, and focus on the certiifcations. Here are the details for the resources for the GCP Certified Professional Cloud Architect exam. The subscription also gives you access to GCP labs and sandbox environments for testing. If you work for a company who covers costs for a professional organization, you may be able to use it for this.
Coursera courses. Google has their official Google Cloud Training courses through Coursera, so the certificate should be valuable. This one is specifically for the GCP Professional Cloud Architect exam.