I run GOG Galaxy on my Steamdeck. Currently it has Proton 9.0-3. I installed Gunhead no problem but when I tried to play the game I couldn't look around using the mouse. After trying a bunch of stuff I found that disabling the GOG overlay for this game let the mouse work correctly. There are other games that also have control problems when the GOG overlay is active so try turning it off if the controls are wonky.