Sparkmage's Gambit deals 1 damage to each of up to two target creatures. Those creatures can't block this turn.
If Sparkmageâ€™s Gambit resolves, but the damage is prevented or redirected, the target creatures still wonâ€™t be able to block that turn.
If Sparkmageâ€™s Gambit has two targets, and one of them is illegal as Sparkmageâ€™s Gambit resolves, only the remaining legal target will be affected. The illegal target wonâ€™t be dealt damage and will be able to block that turn. If both targets are illegal as Sparkmageâ€™s Gambit tries to resolve, it wonâ€™t resolve and none of its effects will happen.