The G.D.O. (aka Garage Door Opener or Remote Code Device), is a Tau'ri device that transmits a signal to the S.G.C. computer, identifying the SG unit so that the SGC can open the iris covering the Earth Stargate. (SG1: "The Fifth Race")
A G.D.O. and iris code (a sequence of numbers that must be manually entered into the G.D.O.) is assigned to every member of every SG team. The codes can be altered if one of the devices is compromised. If a team is captured or too long overdue, their G.D.O.s are locked out of the system, resulting in the team being left permanently stranded off-world, unable to return to Earth. (SG1: "Children of the Gods")
G.D.O.s have also been given to various allies, such as Bra'tac of Chulak and the Tok'ra. (SG1: "Family", "Serpent's Song")
The Atlantis expedition also possesses G.D.O.s, but of a different design. They are assigned to individuals instead of teams, and are coded to a specific user. (SGA: "Rising", "Home", "The Storm")