I can only speculate on the Windows 8 compatibility, but if GOG lists it as compatible, then it should work. Probably best if you don't install in the Program Files folder, but that should be standard with older games these days.
As for the game itself, it sure does hold up. The graphics were fairly weak even back in the day, especially the faces and model details. However, the atmosphere is spot on. Besides, you approach most enemies from behind, so it shouldn't be much of an issue.
The gameplay is excellent, good to this day. It's all about sneaking into places and avoiding guards. You have to keep quiet and stay in the shadows as much as possible. It's not a fast game. There are some modern games that work similarly, but the difference there is that, if you're discovered in Thief, you're pretty much dead. The game is fairly unforgiving like that.
As to the UI, it is fundamentally a standard FPS UI, with mouselook and WASD movement. However, due to the nature of the games, there are other important keys to keep note of and rebind, if necessary. It's a good idea to immediately look at the keybinds for leaning left/right, inventory scroll/use/pickup/drop, environment use (e.g. open doors), run and crouch hold/toggle. There might be other stuff I'm forgetting, but you're probably getting the gist of it. A simulator this is not, be easy :-)
I don't think any of the games have a tutorial, so you'll be finding things out the hard way :P