I presume you have already updated the game, if not refer to the "Terrible condition of the game" thread for help to get that done. (The GOG version comes with all updates).

The first thing to do is to use the Video Setup executable / launch file included in the game directory (this only needs to be done once). Note that for me, any settings above 1920x1080 @60Hz will result in the game launching at 640x480 with minimum details, others report they can set any resolution provided they don't select more than 59Hz.

Additional improvements can be achieved through tweaks in your graphics driver software. I assume this is simple enough in AMD's Radeon software, but for Nvidia users, this is how it works:
Go to Nvidia Control Panel. First select "Adjust image settings with preview", select the 2nd option (use advanced settings), and then select "take me there". Switch to the "Program Settings" tab, and find the game executable in the list (or select it manually via "add").
There, you can choose various settings otherwise unavailable (like AA, 16x AF, high quality texture filtering). Notice each option has an explanation what each setting does (which appears when you point the mouse at the option). For me, "enhancing" works great, overriding not so much). Don't forget to hit "Apply" in the bottom right of the window once you're chosen all your settings. Enjoy!

PS: There may be additional improvements possible by playing with the "config" file in the savegame directory and especially the "tuner" file in the folder where the game is installed, but I didn't try so far. If anyone with more knowledge gives it a try, please let other players know here if you can achieve better draw distance etc.
PS2: If your controller starts causing problems (turning camera when pressing triggers etc), make sure u start the game first, and then switch on the controller once in-game.
The game is old and has some minor flaws, but once it works it's worth all the hassle.