Does lifelink from combat damage apply before or after the damage is dealt?

Lifelink is a static ability that modifies damage as it is dealt. Under CR 702.15b, damage dealt by a source with lifelink causes its controller to gain that much life at the same time the damage is dealt — it is not a triggered ability that goes on the stack afterward.

This is an important distinction: because lifelink is not a triggered ability, the life gain cannot be responded to separately. When a creature with lifelink deals combat damage, state-based actions are checked after damage is dealt, and by that point the life gain has already occurred simultaneously with the damage (CR 702.15a, 702.15b).

This matters practically in combat. If you are at 1 life and your opponent's 3/3 with lifelink blocks your 3/3, the damage is dealt simultaneously in combat. You take 3 damage and your opponent gains 3 life at the exact same moment — neither player 'dies first' or 'gains life first.' State-based actions are then checked once, and both results are assessed together (CR 704.3).

Concrete example: You control a 2/2 with lifelink and are at 2 life. Your opponent attacks with a 4/4. You block. Combat damage is dealt: your opponent's 4/4 deals 4 damage to you, and your 2/2 deals 2 damage to the 4/4 — simultaneously, your lifelink creature causes you to gain 2 life. You end up at 0 life (2 − 4 + 2 = 0), not −2, and state-based actions then cause you to lose the game.

CR 702.15b confirms lifelink is not a triggered ability, so there is no 'window' for responses between the damage and the life gain — they are one event.