I did it all through gaming mode. Works just fine. Bought it for the autumn sale too.

I had to buy the game; then power off your device from steam gaming mode. Before it shuts off, it brings up the rockstar prompt, and has you download all the rockstar stuff. Turn it back on, click play in steam, in gaming mode. Then it brings up the rockstar launcher right there, then you connect your rockstar social account or make a new one, then the game launched just fine.

Desktop mode was not needed for me…

This is a problem. I feel like this needs to be fixed…