That is plain wrong, especially since Black 2 and White 2 are SEQUELS of Black and White.
GSC is a sequel to RBY that's why they're seperate generations. By that logic BW2 is a generation ahead of BW
Not really. GSC were set in a connected region some years after the events of RBY, but they focused primarily on a new region and new Pokemon and only indirectly continuing the RBY story. The Kanto content was essentially post-main story bonus content, making them separate generations.
Black 2 and White 2 take place in the same region with no new Pokemon, just a few new forms of existing ones and directly continues the generation V story making them the first true sequels.
Each generation actually introduces new Pokemon, not just new forms or a new story.