If on CloudFlare DNS, users not able to access application (which is hosted on GCP)

Hello there, as explained in the title, we are having application hosted on GCP, but when using CloudFlare we cannot access the application. If I change to Google DNS it works just fine.

Right now, we see issues are fixed, but we still have customers complaining about it.

Any status updates, issues? Anyone to help?

What port is it running on?