The G.D.O. (aka Garage Door Opener) 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 EarthStargate.
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 on the planet they're on and ultimately unable to return to Earth. (SG1: "Children of the Gods")