I've been looking for a long time for GOG to add Titan Quest to their list of games. Unfortunately, it never happened. Where could I inquire about it?

Except trying to email Nordic (the now publisher) you don't.

No place to inquire, but you can vote for it on the wish list:
http://www.gog.com/wishlist

Well Dotemu is porting the game to android later this year.
http://www.androidpolice.com/2015/06/16/dotemu-will-bring-pc-dungeon-crawler-titan-quest-to-android-later-this-year/

Since every game* Dotemu ports over to android shows up on GoG I'd say chances are pretty good it will turn up here.

*Except R-type, which doesn't have a native pc version.

WTF really? lol

It will be coming here. Nordic (before they pulled their catalogue) had a post somewhere here in the forums where they said it was one of the games they were looking at bringing us DRM-free in the near future (or something along those lines).
It might not be here in the next few months, but I am sure it will come as tremere110 said, sometime soon.

I always get sick of that game by the time I get to Delphi.

I keep waiting too! Like you I'm fairly certain it's going to happen as well. Maybe not today, maybe not tomorrow but someday baby, someday ;)

I'm actually pretty excited for it. I didn't play it first go around, and I passed on my chance to buy it DRM free on Amazon (back before I GoG-ed this was up there with the expansion as a Digital Download; it was eventually pulled and returned with DRM :( ). I'm looking forward to something that (from what I understand) has a "create a class" like Din's Curse (I believe you can pick and choose from a multiple skill trees, selecting the two you prefer as a build) but is a bit more story directed.

Oh, and loot. Lots and lots and lots of loot!

Thom from NG was kind to reply today about Titan Quest coming to GOG:

"We first need to solve the Multiplayer issue and install new servers, then it is probably just a matter of time until it will appear on GOG."

Thanks Thom & NG. :)

Was that recent at all? Source?

Sent PM to Thom 5 days ago, got his reply today. :)

I had a lot of fun with it, but I couldn't get very far beyond the egyptian type levels. Started to feel too repetitive. Is there an Atlantis type level in the game? I think that is as far as I got :P

The 'install new servers' part doesn't sound great, but I think the game supports direct to IP connections so not a disaster.

Prepare your ass then (not the donkey).
At least the bricks are gold XD