Yes, the Book on the Infancy of Jesus is also on the List of Apocrypha, but the infancy narratives of Christ were already part of the Canon of Scripture. So Gelasius' rejection of the book did not mean his simultaneous rejection of Jesus' nativity.

One cannot argue this, however, of his rejecting the book on the assumption of Mary. Her assumption was not part of the Canon, which suggests (not definitively) that Gelasius rejected the very idea of the assumption simply because it had no biblical support.

Pope Hormisdas reaffirmed Gelasius' decree, as you said.

Were these papal actions tantamount to declaring as heresy the teaching on Mary's assumption? In my opinion, yes.