Gail is a third class cherub, known colloquially as a "Cupid." Gail was given orders everyday from Heaven, however with the chaos of the various factions fighting for control of Heaven, the orders became less frequent. It eventually reached the point where Gail was afraid to return to Heaven.
Metatron tells Castiel that the second trial required to close Heaven is to obtain the bow of a Cupid. Metatron has ascertained that Dwight Charles - a bar tender - will be shot by a Cupid's arrow. He and Metatron start watching Dwight, but Naomi arrives and abducts Metatron. Castiel turns to Dean for help. When they see a delivery woman arrive at the bar, they think she will be the one that Dwight falls in love with. However she is the Cupid, and she causes Dwight and his regular customer Rod to fall in love.
Outside the bar, Dean and Castiel tell the Cupid they need her bow, which appears as a mark on her hand. She tells them how chaotic Heaven is, and willingly helps them. It is somewhat implied that retrieving her bow involved cutting off the hand on which it appeared.