Funny... I posted a thread along the same lines in the lounge. I'm ticked off about the price of parts on Ebay. People are asking a mint for parts, I guess that's the way it goes. I was directed to this thread from my thread.
Here's what I can tell you from my experience. Let me preface this by saying... I am not a body man, however, I do have degree in collision repair (it was my backup plan.)
I purchased a set of O.E. fillers from Gbodyparts.com. They are not the O.E.M fillers. The fillers are okay, however they required some work. When I first looked at them I was scared! After some work they came around. Don't expect to buy them and spray them. They come "Ready to Paint," yeah, maybe on my kids beater!!!
First, test fit them to your car. The mounting holes will need to be adjusted. A dremel tool works well for this. Second, (after sanding to remove "primer," you don't have to take off all the primer, just rough it up real well) lot's of 2K primer/filler. You can find this in a spray can at your local parts shop if you want to go that way. The seam is pretty pronounced. 3 coats then sand, then repeat. Once you are happy, topcoat.
I didn't use spray cans on mine, so it only took a couple times to get them correct and hide the seam. I used a Dupont topcoat to finish the job.
Even if you buy these fillers, do the prep work, and send them to a shop for paint you are still ahead of what you would spend on factory fillers. Are they as flexible? No. For the average GN are they a quality part? Yes.
Honestly, I've been a GN owner for 18 years, if I walked up to a car with these fillers I couldn't tell the difference at first glance if they are fit and prepped correctly. If I really look I could tell, but unless you are building a concours car, you will be okay!