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.
Gap said the leaked 30% off vouchers, which were not intended for general consumer use, would be redeemable, but the company has now clamped down on the 60% fake versions.
A spokeswoman for Gap said the company was "disappointed" that its redemption scheme had been abused, and was now investigating the matter.
The news follows a spate of hoax email voucher incidents occurring in recent months that have turned the spotlight on the medium's security issues.
Littlewoods was forced into an embarrassing decision to reclaim 3,000 unauthorised online vouchers worth £25 each in October, after an email discount code originally meant to target only a few people ended up being circulated on chat rooms and retail websites.