bazilisek: Well, the worst selling GOG is now apparently Baldur's Gate 2. That sounds rather fishy.
Not for long. I think BGII is going to beat PS:T if not Arcanum (no! not Arcanum! my precious!).
What people are looking at is the catalogue, sorted "by bestselling". Seems to be a honest accumulation of sales with no differential component whatsoever. No "popular now!", just total sales, accumulating from opening day and until the heat death of the universe.
So Jagged Alliance: Deadly Games is at the top, because all the members who signed up before Sep 20 got it automatically. The three always free games follow, because it takes effort to "buy" these. Tex Murphy was a Christmas giveaway in 2009. Fallout was on the site from
the very beginning (I think). Arcanum was released in Jan 2010 but it's so awesome that its high rank is really not surprising.
EDIT: see, BG2 has already left Shadow Magic in the dust. #141 vs #213 of 248 games total.