I can just copy the expansion files from my Steam copy to my GOG copy and it works fine. You need to own all 3 Ghost Recon titles (Main game and two expansions) though.
Copy "Tom Clancys Ghost Recon\Mods\Mp1" and "Tom Clancys Ghost Recon\Mods\Mp2" folders from a Steam copy of the games to the same path inside your GOG copy and voila.
So if you really are keen for them you can grab the 3 titles on Steam during a sale and then copy the two expansion mod folders to your GOG.com Ghost Recon install.
I confirm this also, the two expansion packs "Desert Siege", and "Island Thunder" will work if you do this. You may want to copy over the three files named: "IT Manual.pdf", "Desert Siege ReadMe.txt", and "Island Thunder ReadMe.txt" also. Copy them into your "C:\GOG Games\Tom Clancys Ghost Recon" main directory.