For those on a Quest who got Gal Gun 2 on sale, I finally figured out how to fix the randomly dropped trigger pulls. If you run the game normally, it uses the Oculus API directly instead of the SteamVR/OpenVR one, and the Oculus API's controls for this game are fucked. Forcing the game to run in SteamVR fixes the controls, although the sticks feel stiff because of large deadzones.

To force the game to use SteamVR, set GalGun2-Win64-Shipping.exe to Windows 7 compatibility. OpenVR Advanced Settings has an experimental setting to suppress the Oculus API but it crashes the game. There might be a better way to do this like a standard Unreal Engine launch parameter, but I'm too tired to keep looking.