Hello guys !
I have one question, a friend bought Divinity original sin enhanced edition on steam.
I want to know, if i buy it on gog i could play with him or not ?

i saw he can link his steam account to GoG and transfer the game to GoG.
But it says : " for a limited duration "
After this limited duration, what happen, he lost the game on steam and GoG ?

No, I think he has to do it now, because he won't be allowed to anymore in a while. But if he does now, it will last forever.

That's how the occasional gog/steam cross-redeem work. You have a small window of opportunity to do it, but when it's done it's done for good.

You can, using the "Direct Connect" option on the Join screen.

I haven't used it lately but i've played with people on Steam in the past.