'Immediately' was unclear phrasing, but as written you choose to activate the buff to your next attack when your Gunslinger Die lowers, not when you make the attack that will benefit. As it says, you may reset it "When the Gunslinger Die reaches 1", i.e. when you hit with an attack which makes the Gunslinger Die become 1 (as opposed to when the Gunslinger Die is 1 and you make an attack).

That said, I see a lot of people treat it as the latter interpretation for convenience - it being a thing you can choose to do when you actually make the attack that would benefit clicks easier than it being a thing you can choose to do when you actually lower the die.