Does a creature with lifelink gain life when it deals combat damage to a blocking creature?

Lifelink is a static ability defined in CR 702.15a: damage dealt by a source with lifelink causes its controller to gain that much life. This applies to all damage the creature deals, not just damage to the defending player.

During the combat damage step (CR 510.1), each creature assigns combat damage. When that damage is dealt simultaneously (CR 510.2), any source with lifelink triggers its life-gain effect at that same moment. It does not matter whether the damage went to a player, a planeswalker, or a blocking creature — the life gain happens regardless of the target.

It is worth noting that lifelink is not a triggered ability; it is a static ability that modifies how damage works (CR 702.15b). This means the life is gained at the same time the damage is dealt, not afterward on the stack.

Example: You attack with a 3/3 creature with lifelink. Your opponent blocks with a 2/2. When combat damage is dealt, your creature deals 3 damage to the blocker and you gain 3 life — even though the damage never reached your opponent.

So whether your lifelink creature is blocked or unblocked, you gain life equal to all combat damage it deals, including damage assigned to blocking creatures.