It wasn't the basis for the Opel, BUT... they are definitely very closely related.
I think it was Bob Lutz who had the idea for a smaller version of a Vette for the European market (at least from a design perspective), and an original Vette designer who started/drove the GT design work. I've read interviews where they've stated this stuff outright. Underneath the GT is a Kadette or some other existing Opel platform (I forget). Interestingly, both cars do share some unusual chassis elements, like the transverse leaf spring, though I'm not sure if that was Vette inspired or already there.
Then if you look at design cues at the time, I'd argue all of these similar-shaped cars are primarily influenced by the 1950's Ferraris, which seem to be the first with these (stunning) lines.