They are very much different. The GH, which happens to be my favorite cherry, is a “traditional” VA based cavendish that is produced with their high quality VA and topped with a very nice “true” cherry flavor. SG Black Cherry is a bit stouter smoke. I believe that, though it is technically a cavendish, that it is a VA that goes through the same steaming process as SG NIght. It is kind of steamed and baked in its own juices. At some point a similar cherry flavor is added. Both are nice and are tobacco forward, not just candy. They are two very different tastes with the SG being a stronger smoke.
BTW, I have only purchased the SG in Europe. Are you based there, just purchasing it from there online, or do you have another source?