By using the site to promote new products related to mobile phones that need a place to create volume of sales.
By doing this Giffgaff could create a community of new products that started out on Giffgaff before moving to the larger market. Giffgaff could be the place for innovators to flourish, whilst challenging the bigger competitor sites in bringing new and innovative products to a market.
giffgaff members would have the first look at the products which in turn could develop into new members joining.
Kickstarter is probably a model for this, so it could either be a kickstarter on the site or develop a totally new service.