If you would like to play these new "Skyrim Roleplay" servers such as Keizaal or Mereth, but don't want to purchase another copy of Skyrim then this article will explain how to get the mods and launcher running properly.
The first thing you should do is make sure you have downloaded the correct version of SkyrimSE. GOG Games has a newer update available than Steam does (1.6.1179 vs Steam's 1.6.1170) but all the mods were made for the Steam version. So ensure you download the 1170 version instead of 1179. The Steam version is prefered, but if you can only get the GOG 1170 version, the next step is for you. If you can get the 1170 Steam copy, then you should be able to install and run and install the mods exactly as intended.
The next step, if you can only find the GOG version is to rename the "Skyrim Special Edition GOG" folder in C:\Users\zutri\Documents\My Games\ to be just "Skyrim Special Edition"
Now you should be able to install the mods normally through Nexus!