Grey Knights have the 'True Grit' ability when armed with storm bolters, enabling them to use them in close combat like lesser creatures would use pistols. This means they gain the bonus attack dice for being armed with an additional close combat weapon, providing they have not charged in the same Assault phase. This rule only applies to Grey Knights in power armour.
Errata — Chapter Approved: Codex: Daemonhunters Q&A (v1.0): Grey Knights have this ability permanently, not only when charged — but, unlike the True Grit entry in other Codexes, it confers no additional benefit when they charge.