Poison layers are considered 1 type of debuff.
Most healers can remove 1 debuff. Since 4 layers of poison is still considered a "Poison Debuff" it counts as 1 debuff.

And yes, Gakido can remove all 4 layers of Poison because as I said, 4 layers are still considered to be 1 debuff.

If we had to remove each layer individually, then Poison Tai would be way too overpowered. I mean, currently, at 4 layers it can tick for 7999 damage...that's already pretty broken.