Resistance to Bludgeoning is all that's required
Resistance is active if a:
...creature or an object has resistance to a damage type,[then] damage of that type is halved against it.
In the case of Reverse Gravity, the damage is described as (with my emphasis):
If some solid object (such as a ceiling) is encountered in this fall, falling objects and creatures strike it just as they would during a normal downward fall.
Falling Damage is non-magical and therefore any and all sources of resistance to Bludgeoning damage are covered with this.
Some creatures may have a more specific resistance like "non-magical bludgeoning", but the umbrella "Bludgeoning" covers all sources.
Some Sources of Bludgeoning Resistance
- Barbarian Rage
- Natural resistance by a Monster
- Non-module specific monsters include Awakened Trees, Swarm of Bats, Swarm of Insects, Swarm of poisonous snakes, Swarm of Kippers, Swarm of Rats, Swarm of Ravens, Treant, Wood woad.
- Stoneskin spell (non-magical only)
Rage gives you resistance to bludgeoning damage. If you take bludgeoning damage from a fall, you resist it