GOG.com has a download client called "Galaxy" and it is somewhat similar to Steam. I recently installed this to see what kind of mischief it might create for the Last Resort mod in Swat 3.
Launching Swat 3:
Galaxy can be used to launch Swat 3 but once you install the Last Resort mod the proper way to launch the game is with the Last Resort Launcher. If you start the game using Galaxy you might have problems with the menu graphics being too large for your screen because the mod may be activated in the background. If you really want to launch the game from Galaxy you have to remember to turn off the Last Resort mod with the handy toggle switch on the Last Resort launcher.
Verify/Repair:
Galaxy has an option to verify the Swat 3 game files and to repair any damaged files by re-downloading them. This feature can damage the mod therefore it is recommended you use the Last Resort Launcher to turn the mod off first. There is a toggle switch at the top right of the Last Resort Launcher that you click and then hit the "Save and Quit" button. If forget to do this you can always re-install the mod after you have verified your game files.
Here is what does get changed when you use the "verify/repair" feature:
- default copies of swat.cfg and swat.ini are downloaded. This is only a minor nuisance since your game's custom key assignments are lost.
- If you replaced the original Swat 3 movies with versions encoded with a different codec (ie XVID) then Galaxy will re-download the original game movies and overwrite yours.
In conclusion the Galaxy download client doesn't harm the Last Resort mod if you turn it OFF first. However I recommend you don't use the "verify/repair" feature unless you actually have a problem.