I know its possible by examples I've seen. But I'm not sure how to do it? Trying to make it when you sit in the seat you get a gui popped up in your face :P
Check by the Welds... there's a SeatWeld thats welded to ur Torso, check Part1 in the Property and then check if the Part1's Parent has a Humanoid, if it does then, get the PlayerFromCharacter and stick the Gui into his PlayerGui...
In a LocalScript in the Gui.
if game.Players.LocalPlayer.Character.Humanoid.Sit == true then script.Parent.GuiName.Visible = true end