When assigning combat damage, you choose for each creature whether you want to assign its damage to the creature blocking it, or if you want to assign it to the player or planeswalker the creature is attacking. You can't split the damage assignment between them.

If either creature is an illegal target as Garruk's second ability tries to resolve, the creature you control won't deal damage to any creature.

Garruk's last ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won't gain the ability.