Yes, this runtime supports arbitrary dependencies specified via a `requirements.txt` file. If you find a library that doesn't work, please let us know. In general, anything that works using an open source Python 3.7 distribution should be supported.
Incidentally, the best way to "let us know" about bugs in GCP (App Engine or whatever else) is via the Issue Trackers, as per https://cloud.google.com/support/docs/issue-trackers (disclaimer: making this and similar processes work is my current job at Google).