Thanks, it shows me how to install it, but like jay wrote and like I just found out, it's not for this version. Meh, no big deal. :D
Oh well, odd that it says it works on the Steam version but then doesn't work on the GOG version, as you'd expect them both to be the latest version executable considering the game is from 2005.
I just read the Gun entry on the WSGF (Widescreen Gaming Forum) site. There are registry entries that can be edited for the screen resolution, so that can be done. However, the result is a stretching of the screen, not showing additional display, so for me that is worse than just playing it pillarboxed for the original 4:3 screen ratio.