If I were me, I would bring a gift. Small as it maybe, I don't think I'd show up empty handed. Even something as small as a nice bottle of wine since it sounds like a casual affair, although I like your mom's idea of the restaurant gift card. The newlyweds throw a reception to thank you for being there for the wedding ceremony, and then you give a gift to thank them for inviting you to the reception. These may be dated rules, but nonetheless, I think a gift is appropriate for the occasion.