Install Gaia on your phone, plug your phone in, select Android Auto on Uconnect, go to app list, select Gaia. Same as selecting any other app in Android Auto.
Well I must be doping something wrong. The map on the screen doe not change to GAIA. GAIA is not listed under apps on the screen. If I run GAIA maps on the phone it is not mirrored. Maybe a problem with my Samsung.
You might need to configure your Android Auto settings. It starts within AA on UConnect, then finishes on your phone. Make sure that Gaia is listed there.
User error, it is working now. I have subscriptions to Trails Off-road and Gaia. so when I'm wheeling next month, the Trails Off-road waypoints and such will be pushed to the Gaia maps. I'm interested to see how this all works together.