Hi!
I bought the Pillars of Eternity game from Steam and I have it installed.
Now I bought the White March Expansion Pass on sale from GOG, but when I try to install it, I get the message "Base game not detected" and "Please install the GOG version".

Is it true that these versions really are incompatible?
Or is there some workaround to this problem so I can use the expansion pass together with my base game bought from steam?
I don't want to buy the base game twice. :)

Yes, they're incompatible. Sadly, no, there's no reasonable way to use the GOG expansion installer with Steam version of Pillars of eternity.

I would suggest contacting GOG, explain them your situation and ask for a refund or at least store credit. Don't forget to mention that you have already downloaded the installers and that the store page only says "Please Note: The base-game, Pillars of Eternity, is required to play.", not that you need the GOG version.

Oh dear, I wish GOG could extend the DRM free principle to make expansions compatible with whatever other base game versions are installed... :(

This is what happens when you buy an expansion disc in a store: it finds the base game and installs itself, plain and simple. I wish it was the same way here!

That's, sadly, currently impossible, especially with Steam. In ye olden days, it was the game's creators who made installers, made sure they're compatible etc. Today, it's often the distributor - Steam or GOG - who makes sure the game gets installed correctly and is compatible with the expansion. Since all of these distributors use their own methods, however, it's not possible to mix them. Proper standardization would solve that of course, but fat chance of that ever happening... Contact GOG via support form, they won't reply you on boards (most likely, rarely they do.)

OK, thank you for your replies Fenixp! I will contact GOG via the support forums.