It is correct that Nanda, Krsna, the gopas, gopis, etc. left Gokula to move to Vrindaban. There is no discrepancy because the whole area of Mathura is known as Vrindaban. Every place there is known as Vrindaban, but there are different quarters.
Some of the Gopis were detained, not allowed to go to Krsna, we accept that the Gopis danced with Krsna anyway, and they went in their spiritual body. Whatever was detained was material body.
Regarding the speeches of Brahma and Nagapatni to Krsna, that is not their quoting, that is my quoting—therefore it is called the Bhaktivedanta purport. But it is not incompatible, as you have suggested, because the Bhagavad-gita existed long before.