cogadh: If by "fix" you mean "created a whole new bug that prevents the game from working at all, but did make that error message go away" then yes, it fixed it. ;)
You're not alone. I ran into a problem with the Dark Engine Resolution Patch. It'll take me some time to sift through the TP2 that is responsible for making the offending modifications.
SparkTR: I have that exact .exe installed with the DDFix and it works perfectly with the gog version, they should bundle it with the game.
You should be able to use another modified EXE without trouble since it utilizes that same resources that GOG provides. It's probably going to be the simplest and most immediate solution, unfortunately. I prefer to do my own modifications for edification purposes, but I understand why others might not want to bother.