The Dell/Gold Key/Whitman story is extremely convoluted. I'll sidestep the Dell part for now and go directly into Gold Key. They were a major publisher throughout the 1960's, sometimes even surpassing Marvel and DC in sales. But by the mid 1970's, the comic book market had changed. Part of it was the economy. Part of it was that the Gold Key audience of younger children wasn't as interested in their product as their older siblings had been.
Unsold Gold Key comics were returnable to the publisher for credit. Someone figured that by publishing comics under the Whitman name and distributing them in a plastic bag would cut down on returns. The Gold Key comics were returnable. The Whitmans were not. The Whitmans usually had three comics per bag but customers could seldom see exactly what they were buying, so the idea never caught on.
Other companies' books were sometimes published under the Whitman brand, including some of Marvel's early Star Wars books. Aside from this small variation, the comics are virtually identical and valued about the same.
