My inReach Mini is not showing up in my devices on Garmin Connect? Despite being synced via inReach and the software showing up to date. And I added it on Garmin Express.

There is nothing on the Mini that would sync via GCM. Everything on the device that needs syncing (including activities) will sync via the Explore app. I've never actually looked to see if it shows up in GCM.

Once upon a time, pairing with Explore required the use of GCM. But it hasn't been that way for a while. AFAIK, that was the only reason to use GCM with iR devices.