The term "super C" is as much a marketing term as anything. These days it generally is used to mean a class C that's built on a MDT chassis, something larger and beefier than the usual E series (or equivalent) van chassis.
The motorhome you're looking at is built on a standard E-450, and is just a large class C. It may well have been called a super C by Gulfstream (or by someone else) but it's not intrinsically different than many class C's not called super C's. Since it's a pretty large motorhome, it would be wise to see how heavy it is and hence how much payload you can carry; some had pretty little weight capacity. (I'm talking generally; I don't know specifically about these units.)
The Gulfstreams were not terrible units, from what I can gather. As with any used motorhome, it's more important to look at the condition and the care it received from the former owner(s) than the brand or initial quality; a well-kept, very ordinary model will be better and more reliable than the best built unit after some time of neglect.
When I was shopping for my motorhome, I looked at a Gulfstream Ultra. I loved the layout, which was quite unusual: instead of a queen bed in the back, it had a pair of bunk beds and a second dinette table, which I think would have been very nice for what I wanted. Unfortunately, the unit I was looking at was in very bad shape with extensive water damage all throughout the unit, so it was a no-brainer to let it pass.