Does Gardens of Becoming ability activate when a unit is already exhausted on the battlefield?

A unit on the Gardens of Becoming battlefield cannot activate its granted ability if it is already exhausted.

Because the ability granted by Gardens of Becoming is [T] : Gain 1 XP, it requires exhausting the unit as a cost. According to Rule 414.4, an action (the cost) must be able to be completed for the cost to be paid. Since a unit that is already exhausted cannot be exhausted again, you cannot pay the cost required to activate the ability.

Note: I could not fully verify this ruling against the retrieved rules.