SCAgent: The widescreen fixes by nemesis which can be found on pcgamingwiki or his site (ps2wide) are perfectly compatible with the GOG versions of Hitman (1-4), I have done multiple playthroughs with them with no problems.
download,unzip, they are password protected the passes are in his site, and copy paste them in the games directory (where the main .exe is) then edit the ini file there to add your resolution and other fixes.
Things to note for the widescreen fixes
HitmanC47 - Has a value to increase the draw distance (no fog) works fine in all levels except the ones in Columbia, I got a pretty good PC but the frames in those levels with the fix go to single digits, decrease the value or use default values.
HItman2SA - for the widescreen to work on the gog version, you have to run upx,exe which is included in the zipfile (dont worry its safe) the .exe needs to be uncompress for the fix to work (its still DRM-Free and the same game as before. there is also crash when you try to climb the tower in "Park Meeting" You don't have to do so, in fact its not a good way to complete the mission, but good to know just in case.
HitmanContracts- Nothing of note.
Hitman Blood Money - Nothing of note.
And another thing:
CAP THE FPS!!!
Use rivatuner or your program of choice, Hitman 1 has physics bug that break the game if played at fps higher than 100+.
HItman 2 will not start at all at high FPS
Contracts is mostly fine but cap it as well just in case
Blood Money has physics problems as well and you can get stuck in the terrain in some levels.
In general its good practice for old games not to have them run with 200fps
Cap it to 60 and you'll be fine.
Hope I helped out. Have fun playing!
I if you have problems, you should download for the Hitman Codename 47 ENB DirectX8 to DirectX9 convertor. Regular version of the widescreen fix don't work with my PC.
Also if you running the the Hitman Blood Money with Galaxy you should disable the in-game Galaxy features to avoid crashes.