Not exactly sure. But like most of these places they usually have both OE gaskets and pattern ones. Unless you absolutely specify they will usually give you the pattern ones.
Also some have been getting the wrong gaskets in sealed sets and whatnot lately it seems. Thats usually the case everywhere though annoyingly, the gasket sets/parts lookup books don't seem that accurate...
About the gaskets, yes if you want GM, say so. I quote GM numbers but they sent non GM parts.
It dosn't really matter, its still from the same factory or standard, just without the GM logo stamped on it. Any factory or manufacture can make a GM pattern part, so long as quilty standards is good and its built to specifiaction, its just as good as a GM part....but without the high price tag.