I prefer to buy games on GOG. I would like to know if I would I be missing out on anything if I buy on GOG instead of steam. I heard some things about mods possibly not working on GOG but not sure if that is still current or if the mods are that important.

I became a member of GOG just over 12 years ago. I have bought many (roughly 200) games since. I'm not one to go in much for mods, but the few I've tried over the years seem to work just fine. I've used mods on Planescape Torment, Masters of Orion, Hearts of Iron, and maybe a few others, with no problems whatsoever. I think that in general, if a mod works on a game on Steam, it will also work on games purchased from GOG.

Stellaris expansions and updates are released on the same day on Steam and GOG.

Anything other than that I can’t comment about, as I never used any mods in all my years playing Stellaris,

The mods works on the gog version of Stellaris (provided of course said mod is compatible with the version of Stellaris you're using, but that's the same issue on other stores)

The main difficulty is that a lot of mods are locked within the steam workshop, fortunately several authors upload their mod on other places so non steam users can use their mods

But it can unfortunately happen that a mod author don't want to do that and want to keep his mod in the steam workshop only, and if you're interested specifically by such mod and don't have steam you'll have to use an external tool to download and edit some file , there are more explanations on how to do it from searching on google.