As far as I know Torque they are different, the GTi is made up of 2 pieces, the 1st is the spoiler and the 2nd piece is the lower splitter section. But the upper section i think is different on a Jetta.
I confused my VW parts guy buy asking too many questions on part numbers earlier, I am not sure if the lower splitter will fit straight on to the Jetta spoiler.
I will find out part numbers and will let you know.
Thanks for looking in