Grayshirt is the callsign of a Viper pilot sortied against two Cylon Raiders that appear during Galactica's travels sometime after the Fleet's reunification and departure from Kobol (Home, Part II).
Grayshirt is not seen, as viewers only see the members of CIC as D'Anna Biers films the attack from the command crew's perspective. Grayshirt and his wingmen destroy the Raiders, but not before they make a secret transmission relay from Biers regarding a Cylon defector and her child to other Cylons (Final Cut).