Newer gaming mice have pretty high resolution (3000 DPI or more), but settings in GZdoom haven't kept up with them. Probably the easiest fix would be to allow mouse overal sensitivity lower than 0.5 in the GUI and to allow finer step than 0.1.
(Yes, I know I can set whatever value I want by editing the config file. Yes, I also know I can effectively lower the sensitivity by fiddling with turning/mouselook etc. sliders. Yes, I know gaming mice utilities support auto-changing DPI for different games. But neither is a "clean" solution.)
To forum mods: I accidentally posted this without logging in, please delete the "anonymous" thread.