GUInterface: Choose the correct monitor from the game's launcher (Riot Engine Configuration), it's under Display Driver.
Seanobi: Don't make the same mistake I did. I was looking under Display MODE (which was blank) instead of Display DRIVER. In my case, I had 1 video card name as 2 different selections. Just select the other one that's not highlighted and start the game. Then you should be good to go.
If anyone has issues, try running The Suffering Settings as ADMIN. That's what I did.
You will have multiple selections with the same name with multiple monitor because Direct3D 9 doesn't give any other information except the name of graphics driver's user mode DLL and the graphics card's name as seen in Device Manager.
The empty display mode list is a bug I fixed in my patch. It basically prevents the game from running on any other monitor except primary one.
https://www.gog.com/forum/the_suffering_series/the_suffering_series_aio_unofficial_patch Running as admin is not required neither with this patch as long as regular users have write access to game install directory, which is the case by default with GOG version. At least I can confirm this if you don't change default install path.