Only creatures you control when Gnawing Crescendo resolves will get +2/+0. However, if a nontoken creature enters the battlefield after Gnawing Crescendo resolves and then dies while under your control later that turn, you'll create a Rat token.
Creatures you control get +2/+0 until end of turn. Whenever a nontoken creature you control dies this turn, create a 1/1 black Rat creature token with "This token can't block."
