Just like other compatible Pokémon, Basculin's ability will change to the regular ability it doesn't currently have. The ability it will get depends on its form. Basculin Red Stripe's abilities are Reckless and Adaptability. Basculin Blue Stripe's abilities are Rock Head and Adaptability. When you use Ability Capsule on either of the two, the Pokémon's ability will change to whatever ability it doesn't have already(excluding hidden abilities).
If you are wondering whether the Ability Capsule allows it to get an ability of the opposite form, the answer is no. Basculin is incapable of changing forms, and cannot have an ability exclusive to other form.
Sources: Knowledge, this, and this
Hope I helped! :)
