Does Google App Engine Flex support Pipfile?
You can just include Pipfile and Pipfile.lock in the .gcloudignore file and run pip freeze > requirements.txt after any library changes.
For App Engine Standard the explicitly state that they do not support Pipfiles and immediately block you from pushing your project if it contains a Pipfile. In searching the documentation, I don't see anything that mentions if Pipfile is supported or not for App Engine Flex.