A Time Gate is a teleportation portal in Chrono Trigger.
The first Time Gate was accidentally discovered by Crono and his friends during Lucca's Telepod demonstration at the Millennial Fair in 1000 A.D. The energy from the Telepod reacted with Marle's pendant creating the portal and sending Marle through time. Crono then followed her through the Time Gate to rescue her.
After successfully rescuing both Queen Leene and Marle in 600 A.D., Crono and his party head back to the Time Gate in Truce Canyon. Lucca uses a device she created called the Gate Key to open up the portal manually. Together, the three of them jump back to 1000 A.D.
Using the Time Gate created by the Telepod, Crono and his friends finally return to 1000 A.D.
After destroying the Dragon Tank and escaping the guards at the Castle, Crono, Marle and Lucca are chased into Guardia Forest by some of the guards and the Chancellor. While trying to get away, they find a new Time Gate. Realizing they have no other option, they use the Gate Key and jump through the Gate not knowing where they'll end up.
The Gate in Proto Dome send Crono and his party to The End of Time, where the Gates appear as pillars of light. After meeting with Gaspar and Spekkio, and getting their magic powers, they enter a pillar of light again to go back to 1000 A.D.
Arriving in 1000 A.D. via a Time Gate in Medina, Crono and his party travel west back to Truce and use the Gate at the Millennial Fair, which then transports them back to the pillars of light at the End of Time. They then use a different pillar to head back to 600 A.D.
That pillar ends up transporting them to the Time Gate in Truce Canyon.
After Crono's party acquires the broken parts of the Masamune, they head back to the Time Gate in Truce Canyon which transports them to The End of Time. They then use another pillar of light to transport them to the Time Gate in Medina in 1000 A.D.
On a mission to find the red mineral needed to repair the Masamune, Crono and his party leave Medina via the Time Gate, travel through a couple pillars of light at The End of Time, and appear in the Time Gate in the Mystic Mountains in the Prehistoric Era.
Crono's party again uses the Time Gate on their way back through the Mystic Mountains.
After having jumped through the Gate in the Lair Ruins, Crono and his friends find themselves in a new era. It's 12,000 B.C., known as the Dark Ages, where a magical kingdom floats high above the surface of the frozen planet. The Cave is where they emerge.
Now finding themselves locked out of the sealed Time Gate, Crono's party travels back to the Gate at the Mystic Mountains and, using the pillars of light at The End of Time, jump back to the Gate in Proto Dome in 2300 A.D.