It means Get it now, without having to bid to win it. Not all auctions have it, and listia rapes you for credits when you post an auction with gin AND IT sells on GIN. basically it allows you to pay a set amount of credits for an item and the auction ends as soon as you push the get it now button.
If someone has listed an item withGIN it means- Get It Now. You can put up the credits they have listed and you automatically win that auction. If a GIN isn't posted and you ask the seller if they can do a GIN, the seller can only post a GIN if there are no bids on that auction yet. If they post their auction originally with a GIN, you can use the GIN at any point during the auction. Hope I didn't confuse you too much! You will get the hang of it. Takes practice, but definitely addictive! I'm going to fan you right now. Welcome to Listia!!