Definitely use it once your FG is stable. No need to rack to secondary if that's what you're asking, just toss em in the primary vessel. Unless you need the carboy space.
I have no idea how much to use but you can always take a beer, add an amount to your glass, and take notes on how much gin you added to your 12 oz beer. Then scale up to 5 gallons and that should give you a place to start.
This gives me an idea to use Rogue juniper gin in a juniper berry oaked pale ale.