For the most part, the Digital Gate is a program that can be run from most PCs to allow for easy passage between the real and digital worlds. Passage through it can be activated with a Digivice or D-3. Meddling by the Digimon Kaiser caused a temporary lock-out to where only the D-3 devices could functionally open it. Television sets found sporadically around the Digital World can be used for the return trip.
Izumi Kōshirō opens a Digital Gate to send Parrotmon back to the Digital World. In addition the Chosen Children make use of them to quickly travel anywhere in the Real World, such as Japan > Digital World > United States.
The Tamers are sent to the Digital World through a Digital Gate opened by the Spiral. Usually a Digital Gate would send everyone to the same place, however the Spiral's influence caused them all to be split up instead.
Hackmon can also open Digital Gates, something regular Digimon aren't able to do.