You need to find a hook to get restaurants to promote GFT themselves to all their patrons. (Which may mean a referral program for restaurants which helps them get diners to come back.)

You need to go with direct sales to corporates (everyone eats lunch). But this can only work if the walking distance area has more than 1-2 participating restaurants.

You need to list on various coupon websites, at least initially. groupon and retailmenot etc.

Really, there have been a lot of startups that have failed in the gift certificate business who have had awesome tech - because their marketing didn't match their tech strengths.