Priority code gamma was a Starfleet code used to decode a priority one secured message.
Captain Jean-Luc Picard used his key access, "4-1-2-mark-8-0-Picard, Jean-Luc, Starfleet priority code gamma" to decode a message from Admiral Haden concerning the Romulan defector Alidar Jarok. (TNG: "The Defector")