Goodmark is not a manufacturer, to my knowledge.
Most of these parts are stamped in China by Golden Legion and others. Typically, these would be the same stamping plants that do the late model crash parts.
Like anything, the time and money spent on engineering and tooling directly affects the quality of the parts produced. The Chinese CAN produce decent stuff, in fact a LOT of late model OE parts are produced there. From what I read in the industry magazines/papers, the key is that you have to closely supervise the resultant product to ensure the quality remains at an acceptable level.
Goodmark, Dynacorn, Ecklers, Tamraz, etc are all just distributors/retailers of panels produced by a handful of overseas stamping plants.
Even as popular a part as 70 Chevelle fenders have had issues with the "repros"...which I believe was corrected by reworking the tooling. Have to figure there are a lot more 70 Chevelle fenders that will be sold than 78-81 Malibu/Camino fenders, so I would assume that the QC isn't as stringent for the latter. As I stated before, last time I tried an aftermarket fender on a Camino, it was not even close to fitting to an acceptable gap.
There are stamping operations here in the US that do vintage car stuff, but I don't think there is enough $$$ in G bodies for them to get interested. They stay pretty busy with tri-5 Chevy stuff, as do the Asian suppliers.