Yes, it's possible. I'm using the Long War 2 mod with the GOG version.
Steps how to install:
1) Create a folder called Mods in the game installation folder. If the mod is for the base game it should be in "XcomGame/Mods" or "XCom2-WarOfTheChosen/XcomGame/Mods" for War of the Chosen mods.
2) Edit the file "DefaultModOptions.ini" which can be found in "XcomGame/Config" or "XCom2-WarOfTheChosen/XcomGame/Config".
3) Add the line: ActiveMods="mod name"
4) Save and launch the game
Maybe add what is the actual name of the mod, noticed that some users add the folder name instead of the file name of the mod.