Hi
Does the GoG version contain the total conversion mod or any enhancements actually? If not are there any problems installing the TCM from Moddb and do I need to install the multiplayer patch?
Thank you for your help.

Nevermind :o) I think I've found what I need here - https://steamcommunity.com/sharedfiles/filedetails/?id=1208640466

GOG provide two version - GOG (offline) and GOG Galaxy. Only galaxy patched same as Steam. So offline version work with any mods. Galaxy - not.
Never try install mods on Steam version. I think we have offline\Retail for that, m? Do you try this guide and multiplayer? Online still works? I dont think so...

For 1.3 unofficial patch, use version r130. You can find it on moddb nowadays. It's made for the GOG/Steam versions with multiplayer.

The conversion mod does indeed work, but you need to install it in a cycle.

1. 1.3 installer
2. Conversion mod.
3. Conversion mod patch. After this you should probably install things like Dark Times 2 that also bundle 1.3 with (pay no mind to the talk about the HUD, you don't get the gaudy blue HUD after you're done here).
4. 1.3 installer again. Yes, really.
5. Anything else you want. I would suggest this mod for a menu UI that matches your game's resolution: https://www.moddb.com/mods/star-wars-battlefront-ii-full-hd-interface

Alternatively, there's a BF1 conversion pack by Marvel4 that just contains the maps and XBOX DLC, and doesn't use any of the above to work at all. It's pretty easy to track down nowadays. This may be preferrable to you even if you do install 1.3, as the bigger one does tend to contain items that are not BF1 related at all (including in the BF1 maps).