Ability Shield + Gastro Acid

The Ability Shield is a held item introduced in Generation IX.

Bulbapedia has an open question regarding this item: If a Pokémon whose Ability has already been suppressed by Gastro Acid obtains an Ability Shield, is the Ability suppression removed permanently (i.e. even if the Pokémon loses the Ability Shield), temporarily (i.e. only while holding the Ability Shield), or does it remain in place?

Fortunately, I have found the answer to this question.

My Pokémon are a Flygon with the ability Levitate and a Seviper holding an Ability Shield knowing the moves Gastro Acid, Switcheroo, and Earthquake. I am using an NPC trainer in Blueberry Academy to engage in a double battle, so my two Pokémon can interact with each other. This was tested on Pokémon Scarlet (Ver 4.0.0) (Running on Nintendo Switch 2)

Turn 1

Seviper uses Gastro Acid on Flygon. Flygon's ability is suppressed.

Turn 2

Seviper uses Switcheroo on Flygon. Flygon obtains the Ability Shield.

Turn 3

Seviper uses Earthquake while Flygon is holding the Ability Shield. Flygon was hit by Earthquake. If Flygon's ability was no longer suppressed, it wouldn't have been hit with Earthquake.

Conclusion:

If a Pokémon obtains the Ability Shield after its ability is suppressed by Gastro Acid, its ability will remain suppressed.