It works surprisingly well! I beat the game with my second character on the Deck exclusively.

The only issue I have with GD and a controller is difficulty targeting specific foes and precuse use of movement abilities. Nothing gamebreaking, just annoying.

It works great, but needs a bit of tweaking. Targeting distant foes and picking up specific loot from a pile of items can be tricky sometimes, but nothing remotely game breaking.

I personally changed the touch pad tu function as a mouse for inventory management.

For comparison, I have a few hundred hours in Diablo 3 (PS4 version) which have a great controller scheme and for me it's the best controller experience of an arpg on the market so far. GD its not far behind, but you might need to personalize it.

I have bought GD specifically because someone said it was better on controller than PoE and I kinda wanted to play on the couch when I was sick a while ago.

At least for a couple of hours as a necromancer it worked pretty well.

For now I went back to PoE with K&M, but I don't rule out, that I might give GD another shot.
And if I do, I will try again with the controller.