So for the PC version of GUN, devs just ported from the PS2 version, hence the very low res textures and almost no graphical settings to speak of, and also the game inherits the PS2 version's 30 fps cap, but this cap can be safely increased without much issues (have only tested 60 and 90 fps so far, and haven't found much issues, but feel free to report below your findings at higher framerates). Here's how:

1. Download a hex editor like HxD.

2. Open the editor, go to "File" and click "Open...".

3. Choose Gun.exe.

4. The first patch will change the game speed to not make the game run too fast when the FPS is higher than 30, so it runs at the correct speed. Press Ctrl+F, go to the "Hex-values" tab and paste "C7 05 C8 3E 6B 00 1E 00 00 00" (without the quotes) in the text area, select the "All" radiobox under "Search direction", and click "Search all".

5. Change the "1E" part for "3C" if your desired FPS is 60, "5A" for 90 FPS, "78" for 120 FPS, "90" for 144 FPS, "A5" for 165 FPS, "F0" for 240 FPS, "17C" for 380 FPS, or "1F4" for 500 FPS.

6. The second patch actually increases the game's FPS lock. Do the same as (4), but instead of pasting the hexcode above, paste this one: "48 53 6A 1E 52 50" (without the quotes).

7. Do exactly the same as you did in (5).

Just to say -- this breaks physics in some areas.
The mission where you have to defend the carriage with jenny can break in parts, specifically when shooting the TNT barrels from the top of the carriage - it will not fully put you on the carriage and you will take full damage when the barrels explode and you will die.
if it happens, just save, exit and run the 30fps limited exe to get past the part that's breaking and go back to the other after.

Ok so I followed the steps of this post and made the game stop working completely lol. I can't even launch it and changing the Hex- values back doesn't do anything too. So now I'm stuck with a game that doesn't even work no matter what I do, great !