A Gate is a hyperdimensional, space-time distortion that allows people to move through time. The first Gate in Chrono Trigger is opened by the Telepod interacting with Marle's pendant, sending her into the past. Crono uses the same means to follow her. Lucca creates a Gate Key that can open any Gate, enabling the group to travel through time at will. More Gates are discovered as the group continues their journey, set up by some unknown entity to put them on course to fight with Lavos and prevent the destruction of the world. After obtaining the Epoch from Belthasar in the Future, the Epoch's Time Gauge locks onto the temporal coordinates of all the Gates, allowing it to travel to any time period accessible by the Gates (Prehistory, Antiquity, Middle Ages, Present, Apocalypse, Future, End of Time).
Time Gates in Chrono Trigger
With the exception of only one, all Gates connect the same geographic location at two different points in time.
After the Proto Dome Gate is crossed by four characters, the Gates to Medina, Proto Dome, and the Mystic Mountains are available at the End of Time. In order to make the other gates accessible, they must be used after these gates have been unlocked for them to link to the End of Time.
The Gate in the Mystic Mountains does not have a corresponding geographic exit in another time period, at least not in-game. In Chrono Cross, it is discovered that this Gate is one of the oldest in existence, as it permitted Leah to travel from her own time in Prehistory to Gaea's Navel, only to return there at the end of the game.
The Gate in Fiendlord's Keep is a temporary Gate, created by the premature awakening of Lavos. It grows to massive proportions, swallows up the Keep, and flings the heroes across time. It deposits Crono and company in Prehistory, near Ioka Village. Magus, however, is sent to sometime prior to the Gate's opening in Antiquity.
The Gate in the bucket at the End of Time takes the group directly to the fight with Lavos. This battle starts with the semi-final fight against Lavos' shell. Once they make it inside the shell, there is another Gate that allows them to return to the End of Time to recuperate. Afterword, the bucket Gate will return them to the inside of Lavos' shell. If the Dimensional Vortex is completed, the Gate in the bucket will instead take the travelers to the Time's Eclipse.
The red Gate at Fiona's Shrine is a sidequest after completing the Sunken Desert. Using it, Lucca travels ten years into the past to prevent the accident that leaves her mother unable to walk. This is the only sidequest in the game that can be failed without ending the game. The only difference it makes is whether or not Lucca's mother, Lara, is able to walk for the rest of the game.
In a New Game Plus, the fight with Lavos is available immediately. Once Crono and Marle go to Lucca's demonstration, there is a spark in the right Telepod that can be activated to create a Gate that travels directly to the fight with Lavos, and is one of the ways to see the director's ending of Chrono Trigger.
In the Nintendo DS version, there are other time Gates in the year of 600 AD and 65M BC that lead to the Lost Sanctum. Likewise, there are Gates in 1000 AD, 2300 AD, and 12000 BC that lead to the Dimensional Vortex.An very interesting occurrence with the time Gates is that an unstable Gate tends to send someone to a certain period of time before that Gate becomes active in the game:
Following the destruction of Lavos and the return of the heroes to their native time eras, the time Gates began to weaken and begin to close.
Time Gates in Chrono Cross
Only one Gate remains active by the year 1020 AD., and it is located at Opassa Beach. It connects ten years into the past and is used by Kid to rescue Serge from drowning. Doing so splits into two dimensions, one where the child Serge survived, and another where he drowned.
Due to his nature as an unnatural singularity - an individual who, through unnatural means, has no double in the other world - Serge has a different effect on the Gate. He is able to use the distortion left behind by this Gate to cross between his world and the one in which he no longer exists. Later on, using the Chrono Trigger, he is able to cross into the Darkness At The End Of Time.