MaridAudran: Out of the Thief: Gold only-missions, one is excellent ("Song of Caverns"), one abysmal, ("The Mage Towers", think "Super Garrett Brothers" with all the ridiculous platforming), and one in-between ("Thieves' Guild"), which is a decent pure infiltration mission, of which most players thought there was a paucity of in the first Thief game before the rest of the plot missions focus on undead tombs and various monster-infested crawls.
You can actually download two of the Thief: Gold missions independently as "fan missions" to play in the T2 engine, specifically "Thieves' Guild" and "The Mage Towers," through your thief FM client-of-choice; I recall it used to be Darkloader for the longest time, I don't know what it is now.
It's a pity that nobody converted the best of the three Gold-only missions, "Song of Caverns," because that's the only one I think one really misses out on by playing The Dark Project. However, it's *not* worth purchasing the game over again if you already own TDP. As echoed here, there are some cosmetic changes to the existing levels and the plot is altered slightly to accommodate the new missions as they segue into the raison d'etre of Garrett. While most of the tweaks are unobtrusive or welcome (like in "The Sword", without spoiling too much), some others were decidedly ill-thought out. The changes in "The Lost City" rudely stand out in my mind as wholly unnecessary and actually detracting from the enigmatic, forgotten nature of the place.
I kind of liked the mage towers-it's a really huge and varied level. It was more memorable than the Thieve's Guild, for sure, though I remember that also being quite long. However, I do agree that Song of the Caverns is easily the best of bunch. Probably one of my favorite Thief levels overall, too.