A little bit of an older thread, but worthy update.
As I believe someone already mentioned, GC basically charges dealer prices. I called today to get a replacement baggage door (got hit with debris in a wind storm and put a nice hole in it). The customer service person I spoke to gave me the phone number to Lippert's parts department (574-537-8900) and told me to call them if I wanted to "save a couple of hundred dollars".
GD was going to sell me the door for $455.82 (including tax and shipping). Lippert sold it to me for $254.53 (including tax and shipping). GD has their own part number for the part (561123), but the label on the door also had Lippert's part number for it (V000613800) - you just need to know what to look for.
So if you know the Lippert part number, you can save a few dollars. Even if you only know the GD part number, call the GD phone number and ask them for the manufactures part number. The guy I talked to offered it up without me even asking for it. It was the same number I already had, so he just confirmed it.