Galarian Zapdos V

Any effect that alters the cost of a Pokemon's attacks is intended to alter the cost of ANY attack that Pokemon uses, not just the ones printed on the card. This means any attack that the Pokemon is given access to, such as Earthen Seal Stone's "Star Gravity" attack when attached as a tool, can be used at the altered cost when effects such as Galarian Zapdos V's "Fighting Instinct" Ability are in play. For example, Fighting Instinct says, "This Pokemon's attacks cost less for each of your opponent's Pokemon V in play"; however a more accurate description would be, "Attacks that this Pokemon uses cost less for each of your opponent's Pokemon V in play." This applies to effects that reduce or increase attack costs, and goes all the way back to the Black & White series thru current series.

If I have Galarian Zapdos V with "Fighting Instinct" Ability active and an Earthen Seal Stone attached, and my opponent has 3 Pokemon V in play, can Galarian Zapdos V use Earth Sealing Stone's "Star Gravity" attack for free?

Yes, you can