In which file did you encounter the issue?

https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/appengine/standard_python37/django/mysite/settings.py

Did you change the file? If so, how?

Switch from PyMySQL to mysqlclient

Describe the issue

When I deploy to GCP using gcloud app deploy, I get the 502 error after deployment. I checked the error with gcloud app logs tail -s default, and the error message was mysqlclient 1.3.13 or newer is required; you have 0.9.3., where 0.9.3 is the version of PyMySQL version in the current requirements.txt. I switched to mysqlclient, and now the error is gone. Also, now that you're no longer using PyMySQL, you can now change the Django version to 2.2+.