Seller can not do GIN until they have at least 5 good feedbacks as a seller and have their wings people and once a bid is placed GIN can not be done, nor can the # of days be changed per Listia rules.
You don't have a seller feedback rating of at least 5 so the GIN option has not been unlocked for you yet. When you get your seller wings you will be able to do GIN. You might not know what a fair Gin is until your more familiar with how bidding goes. You tend to get a lot more credits when you offer free shipping too.