The email vouchers, which are doctored versions of Gap's authentic vouchers offering 30% discounts, were originally circulated to the company's business associates and friends.
Counterfeit versions, which have been altered and rescanned to show a 60% discount, started to appear in UK Gap stores after consumers presented them to staff.

Some comment from BBC:

http://news.bbc.co.uk/2/hi/business/7109118.stm