GIN is GET IT NOW...a set amount of credits you set and the bidder wins immediately. Usually offered with free shipping! I don't think you can set a GIN when you've already got bids on an item.
GIN stands for "get it now", when you post an auction you can start the bids at what you want but you can also list it with GIN ( usually a much higher amount) and if someone really wants it they pay the GIN and win it automatically.