I also had this problem on my iPhone (13.5.1) and the Global Protect app. After successfully establishing a connection to my company network most websites would not load and displayed a certificate error due to SSL decryption. Here is the resolution:
1. Verify that your certificate profile is showing up under Settings > General > Profile. You should have been prompted to install this when connecting to GP for the first time.
2. This is the part that I was missing. Settings > General > About > Certificate Trust Settings (scroll to the bottom).
3. "Enable Full Trust" for your GP root certificate.
Once I did this I could browse to any site without certificate issues. Good luck!