I went to Mod Nexus to get the tonnage limits removed (utter idiocy in this game), but neither mod there seems to work. The proper .pak files are in my pak folder. Anybody had any luck with pak mods in the GOG version? I hear it works in Steam.

The mod is working - I'm using it. You need to put it in the game mod directory, then activate it in the game.

Mods should go into your MW5Mercs\Mods folder. If the Mods folder doesn't exist, you can just create it but make sure you capitalize Mods. The NoTonnageLimit mod should then go into that folder so that you have a folder structure like this:

MW5Mercs\
....Binaries\
....Content\
....Mods\
........NoTonnageLimit\
............Paks\
............Resources\
............mod.json
....Plugins\
....Releases\

That seems to have done it. The tutorials I was looking at said to just put the pak file in the pak folder.