I've found a consistent command to play Google Play Music Stations

"Hey Google, play [station name] music station."

This has worked so far for every GPM music station I have tried. Changing the wording even slightly will cause my mini to play something entirely different... Hope this helps someone else out there.

Edit: After trying several more stations I'm afraid to report that this does not work for 100% of them. I don't understand why GPM Station integration is so horrid...