I would only buy body parts from VW.
Its up to the individual, however personally i have seen consistently poor quality parts from GSF over a period of a few years, enough to convince me not to use them again. Also actually working on my own car, and lots of other people's, i am well qualified to judge the actual quality of the parts compared with parts from the dealer.
However i tried Euro Car Parts last year for my Dad's Audi 90 quattro, and the parts were of top quality.
Not just any parts, we are talking about a set of hydraulic tappets, a front wheel bearing, just some bits for example. If these parts were of sh!t quality then i would of certainly known about it by now.
I'm not actually trying to stop people using GSF, its not my problem, its up to you lot. But don't say you haven't been warned.