Does lifelink apply when a creature deals combat damage to another creature or only to players?

Lifelink is a static ability that causes damage dealt by the creature to also cause its controller to gain that much life. This applies to any damage the creature deals, not just damage to players. The relevant rule is CR 702.15b: "Damage dealt by a source with lifelink causes that source's controller... to gain that much life."

So when a creature with lifelink deals combat damage to a blocking or attacking creature, the lifelink controller gains life equal to the damage dealt — just as they would if the damage had been dealt to a player or planeswalker. There is no restriction in the rules limiting lifelink to damage against players.

It's worth noting (per CR 702.15a) that lifelink is a static ability, not a triggered one — the life gain happens simultaneously with the damage being dealt, not afterward. This distinction matters for things like state-based actions.

Concrete example: You attack with a 3/3 creature that has lifelink. Your opponent blocks with a 2/2. When combat damage is dealt, your creature deals 3 damage to the 2/2 blocker. Even though the damage went to a creature (not a player), you gain 3 life immediately from lifelink.

The same principle applies to non-combat damage as well — if your lifelink creature deals damage via a spell or ability, you gain life from that too, per CR 702.15b.