hi i want to buy the expansion fo grim dawn here but i have the base game on steam it is possible?tyu

No, you need to own the base game on GOG to use the GOG version of the expansion, you would be better off buying the expansion off of steam in this case.

I think the game already was at least once on GOG connect. You could wait until it appears again (and import it to your GOG library) or you have to buy both as the expansion is not standalone.