Manjuice Nutella
Arcane
I've never played a Thief game before.
Then do it. Srsly.
Ok. I'll go play Thief 1 from GoG. Thief Gold, right?
Is there Dark Project to choose from as well?
Thief Gold has three additional levels. It also introduces new things to the existing game like enemies such as Fire Shadows and mages and slightly revamped levels (Bonehoard and Lost City) and not exactly for the better. Shoehorned, if you ask me.
Basically: Tried to fix things that weren't broken. Like I said and explained above, I prefer the DP version. It's imo more consistent and well-paced.
I'd say play the Dark Project and Thief Gold, or at least those additional levels, to see the differences and form an opinion for yourself.
I think you can restore those revamped missions to their original state with some mods or console commands if you don't want to buy the same game twice.