Cruel Celebrant

Whenever this creature or another creature or planeswalker you control dies, each opponent loses 1 life and you gain 1 life.

Deathgreeter

Whenever another creature dies, you may gain 1 life.

Archivist of Oghma

Flash
Whenever an opponent searches their library, you gain 1 life and draw a card.

Hallowed Priest

Whenever you gain life, put a +1/+1 counter on this creature.

A-Druid Class

(Gain the next level as a sorcery to add its ability.)
Landfall — Whenever a land enters under your control, you gain 1 life.
{2}{G}: Level 2
You may play an additional land on each of your turns.
{2}{G}: Level 3
When this Class becomes level 3, target land you control becomes a creature with haste and "This creature's power and toughness are each equal to the number of lands you control." It's still a land.

Celestial Unicorn

Auriok Champion

Protection from black and from red
Whenever another creature enters, you may gain 1 life.

Grim Feast

At the beginning of your upkeep, this enchantment deals 1 damage to you.
Whenever a creature is put into an opponent's graveyard from the battlefield, you gain life equal to its toughness.

Elas il-Kor, Sadistic Pilgrim

Deathtouch
Whenever another creature you control enters, you gain 1 life.
Whenever another creature you control dies, each opponent loses 1 life.

Bloodthirsty Aerialist

Flying
Whenever you gain life, put a +1/+1 counter on this creature.

Curse of the Forsaken

Enchant player
Whenever a creature attacks enchanted player, its controller gains 1 life.

Fleshtaker

Whenever you sacrifice another creature, you gain 1 life and scry 1. (Look at the top card of your library. You may put that card on the bottom.)
{1}, Sacrifice another creature: This creature gets +2/+2 until end of turn.

Contemplation

Whenever you cast a spell, you gain 1 life.

Authority of the Consuls

Creatures your opponents control enter tapped.
Whenever a creature an opponent controls enters, you gain 1 life.

Dina, Soul Steeper

Whenever you gain life, each opponent loses 1 life.
{1}, Sacrifice another creature: Dina gets +X/+0 until end of turn, where X is the sacrificed creature's power.

Archangel of Thune

Flying
Lifelink (Damage dealt by this creature also causes you to gain that much life.)
Whenever you gain life, put a +1/+1 counter on each creature you control.

Campaign of Vengeance

Whenever a creature you control attacks, defending player loses 1 life and you gain 1 life.

Epicure of Blood

Whenever you gain life, each opponent loses 1 life.

Ajani's Welcome

Whenever a creature you control enters, you gain 1 life.

Blood Researcher

Menace (This creature can't be blocked except by two or more creatures.)
Whenever you gain life, put a +1/+1 counter on this creature.

A-Blood Artist

Whenever Blood Artist or another creature dies, target opponent loses 1 life and you gain 1 life.

Bishop of Wings

Whenever an Angel you control enters, you gain 4 life.
Whenever an Angel you control dies, create a 1/1 white Spirit creature token with flying.

Cliffhaven Vampire

Flying
Whenever you gain life, each opponent loses 1 life.

Ajani's Pridemate

Brutal Hordechief

Whenever a creature you control attacks, defending player loses 1 life and you gain 1 life.
{3}{R/W}{R/W}: Creatures your opponents control block this turn if able, and you choose how those creatures block.

Heliod, Sun-Crowned

Indestructible
As long as your devotion to white is less than five, Heliod isn't a creature.
Whenever you gain life, put a +1/+1 counter on target creature or enchantment you control.
{1}{W}: Another target creature gains lifelink until end of turn.