Gateway was a star system that housed at least one planet, also named Gateway. The system had been put under Federation quarantine to avoid mass knowledge of the Guardian of Forever time portal on the planet's surface.
Despite the quarantine, some members of the Klingon Empire knew of the Guardian's existence, including B'vat and Kardok. On stardate 83738.2, B'vat's subordinates kidnapped Miral Paris and took her to the Gateway system, where B'vat took his flagship to the year 2270 to use her to cure the lasting effects of the Klingon augment virus.
The USS Leviathan and IKS SuvwI' pursued B'vat in the past, only to find the USS Enterprise tied up with several Klingon vessels in the system. After assisting the Enterprise to ensure the ship was not destroyed, they had a brief conversation with Spock, who surmised that they were from the future. Saying their farewells, they left the system to track down the Worvig.