Have

Ajani's Pridemate

Whenever you gain life, put a +1/+1 counter on Ajani's Pridemate.

Plains

Don't Have

Inspiring Overseer

Flying
When Inspiring Overseer enters the battlefield, you gain 1 life and draw a card.

Impassioned Orator

Whenever another creature enters the battlefield under your control, you gain 1 life.

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.

Resolute Reinforcements

Flash
When Resolute Reinforcements enters the battlefield, create a 1/1 white Soldier creature token.

Authority of the Consuls

Creatures your opponents control enter the battlefield tapped.
Whenever a creature enters the battlefield under an opponent's control, you gain 1 life.

Griffin Aerie

At the beginning of your end step, if you gained 3 or more life this turn, create a 2/2 white Griffin creature token with flying.

Healer's Hawk

Getting

Voice of the Blessed

Whenever you gain life, put a +1/+1 counter on Voice of the Blessed.
As long as Voice of the Blessed has four or more +1/+1 counters on it, it has flying and vigilance.
As long as Voice of the Blessed has ten or more +1/+1 counters on it, it has indestructible.

Raise the Alarm

Create two 1/1 white Soldier creature tokens.

Cleric Class

(Gain the next level as a sorcery to add its ability.)
If you would gain life, you gain that much life plus 1 instead.
{3}{W}: Level 2
Whenever you gain life, put a +1/+1 counter on target creature you control.
{4}{W}: Level 3
When this Class becomes level 3, return target creature card from your graveyard to the battlefield. You gain life equal to that creature's toughness.

Felidar Sovereign

Vigilance, lifelink
At the beginning of your upkeep, if you have 40 or more life, you win the game.

Daxos, Blessed by the Sun

Daxos's toughness is equal to your devotion to white. (Each {W} in the mana costs of permanents you control counts toward your devotion to white.)
Whenever another creature you control enters the battlefield or dies, you gain 1 life.

Ajani's Welcome

Whenever a creature enters the battlefield under your control, you gain 1 life.

Castle Ardenvale

Castle Ardenvale enters the battlefield tapped unless you control a Plains.
{T}: Add {W}.
{2}{W}{W}, {T}: Create a 1/1 white Human creature token.

Nyx-Fleece Ram

At the beginning of your upkeep, you gain 1 life.