Some of my friends have it on steam and was wondering if i'd be able to play them if i got the GOG version. I'm assuming it connects to a paradox account so i'm kind of assuming it's a yes, but would appreciate if someone can confirm.

I can tell you the Galaxy version does... I'm in Australia and my mate lives in New York but I assume the cheep versions don't allow it

Sounds to me like it does.

I don't know, if this is working for you already,
but I own Stellaris + all DLCs here on gog an a friend owns it on steam
and playing together is no problem. We both live in germany, if this matters.

right now the only way to play with users on steam is to make sure they are opted into the beta version on steam. otherwise the versions don't match up.

im assuming this will be fixed tomorrow when the next big patch hits and they go back to being on patch parity.