It's what Thief 1 should have been apparently. There never was an expansion for the game, but the Gold edition introduced three new missions, like an expansion pack, for the story and changed some levels(I think there was some extra areas and at least one new enemy).
You don't necessarily need Thief Gold for the missions(they've been done as fanmissions for normal Dark Project already and Thief 2 as well), but I'm aiming for completeness. Not to mention, the missions are not afterthoughts, but integrate in to the original campaign and story.