These small palm sized machines were similar to the GDO of the Tau'ri and were used to send a signal which was received by the shield emitter that surrounded certain Goa'uld controlled Stargates. When transmitted on a set frequency, the shield shuts down allowing for entry through the gate otherwise travellers risked being killed as they did not reform on the other side.
Master Bra'tac used a Goa'uld GDO with a set frequency to lower the shield surrounding the Stargate to the planet Erebus when he was attacked. An Ascended Daniel Jackson witnessed this event and when he returned to his corporeal form, managed to recall the code of the GDO which allowed Stargate Command to bypass the planet's shield in order to mount a rescue operation. (SG1: "Orpheus")