Pokemon in a horde battle has a chance of having the hidden ability. So go to Route 19, use Sweet Scent/Honey, then start digging through those Gligars for Immunity. (Which will become Poison Heal upon evolution)
I use Toxic on the Gligars to check them. But things like Trace, or moves that change their ability (Worry Seed, Simple Beam...etc) can also work.
Females will have a chance of passing down the Hidden Ability, and Males breeding with Ditto can also pass it down. Make sure to check your offspring to make sure the Hidden Ability is passed on.
Like I said, I use Toxic to check it. If it has Immunity it will show and be immune to Toxic. Otherwise, things that change target's ability will also reveal what the Gligar's ability is before changing it.