GTA IV: Ballad of Gay Tony is listed as silver on protondb, meaning 'works with tweaks', so that sounds about right. Things obviously aren't perfect. That last 10% of games is a massive pain, since every game is different (all the ones that are well behaved are in the first 90% that already work).

GTA IV even on regular old Windows is a shitshow to get working to be fair, so I'm not too surprised that it's not great on the deck