In the world of System Gate, a Gate Master is a creature from another planet that has Linked with a Human. This Human becomes their channeler to help direct and release excess Source Energy that has built up in the Gate Master.
In a certain context, one can think of a Gate Master as making the human an official System Gate player. A human with a Gate Master is able to use the Gate Master’s Source to power a warp gate and bring units directly into battle. Without a Gate Master, a human can still play and enjoy the sport, but without being able to summon the real Units into battle.
A Gate Master can choose to be with their Human Channeler at all times, or they can live somewhere else only appearing when needed for a round of System Gate. Most hardcore competitors in the sport will live with their Gate Masters. For example, the entire Kisaragi family lives with their Gate Master units, even Grandma Kaedae.
An example of the opposite would be Gregory and Bonnie. Bonnie doesn’t spend most of her time on Earth and instead lives on Jupiter away from Gregory. This is because she finds the environment of Earth to be suffocating and unpleasant.