muttly13: Probably because its pretty new, at least when compared to most GOG games and runs fine on Win7. I picked it up fairly recently from Steam I believe, in some .99 cent deal for all the releases or some such.
I don't think it's that. Titan Quest was released in 2006 and the expansion in 2007. So it is old enough for GOG given the fact they already have other 2007 games. And the fact that it works on win 7 as it is it's a great advantage for GOG as they don't need to tweak it.
Most probably they thought TQ is not so exclusive being sold on all other digital distribution sites. Which is a shame because I prefer having all my games on GOG. I for instance don't use other DD sites.
I still hope that some day they will realize their omission...