If the target creature is an illegal target by the time Moment of Craving tries to resolve, the spell is countered. You won't gain 2 life.
The target creature will still be on the battlefield when you gain life, even if its toughness has been reduced to 0 or less. Any abilities it has that interact with gaining life do so as appropriate. If any abilities trigger on you gaining life, the creature will be put into its owner's graveyard after that ability triggers but before it resolves.