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Reading the digital manual, it says Mysteries of the Sith doesn't need Jedi Knight installed, but you need its disc when installing MotS.

Because of this, does the GOG version (which provides separate installers for JK and MotS) require JK installed to use MotS, as it is listed as DLC? Or can MotS be installed without JK?

More out of interest really as I'll play through both (I couldn't finish MotS when I first played it a good many years ago). Wondering what the best installation option is.
This question / problem has been solved by GRVimage
not sure on that one as I always keep DF2JK installed *but* I did have what im guessing was a pretty rare problem when I was playing MotS on STEAM and that was if I had the JK disc in the drive, MotS wouldn't run. Worked just fine when I removed it though so don't panic if that happens
It appears that it is standalone. The installers for JK and MotS even use different installation folders.

Now if only I could mark this post as the solution.
as they say

GOG helps those that help themselves
GRV: as they say

GOG helps those that help themselves
And as that is what I did, that gets the solution. :D