Does Smite replace Guardian Angel effect?

No, Smite does not automatically replace the Guardian Angel effect.

When a unit equipped with Guardian Angel would die—whether from combat damage, a spell like Smite, or any other effect—you are faced with two competing replacement effects:

1. Guardian Angel's Effect: "If I would die, kill Guardian Angel instead. Heal me, exhaust me, and recall me."
2. Smite's Effect: "If it would die this turn, banish it instead."

According to Rule 372 and the RiftJudge FAQ #10635, when multiple replacement effects apply to the same event (in this case, the unit "dying"), the owner of the affected unit determines the order in which those replacement effects are applied.

The Interaction

• If you choose to apply Guardian Angel's effect first: The Guardian Angel is killed, and your unit is healed and recalled to your base. Because the unit is saved by the replacement effect and never actually "dies" or enters the trash, the "banish" replacement effect from Smite no longer has a valid event to modify and will not occur.
• If you choose to apply Smite's effect first: The unit will be banished, and Guardian Angel's effect will not apply (as the unit is now in the Banishment zone, not dying).

Summary: You are the controller of the unit; therefore, you have the choice of whether to apply Guardian Angel's protection first to save the unit from being banished by Smite.