In some unusual cases, you can gain life even though your life total actually decreases. For example, if you are being attacked by two 3/3 creatures and you block one with a 2/2 creature with lifelink, your life total will decrease by 1 even though you’ve gained 2 life. You’ll put two +1/+1 counters on the enchanted creature.

In a Two-Headed Giant game, life gained by your teammate won’t cause the ability to trigger, even though it causes your team’s life total to increase.