The Forever Gate originated as a teleporter built by Valeria Richards using many kinds of technologies, including the telepods operated by the Griever at the End of All Things, the Kree Omni-Wave Projector, and the Krakoan Gateways. However, she hadn't figured out a way to power it up.
When the alien Cormorant scoured the Earth in search for a mysterious item, he targeted Container Zero, a lockbox hidden beneath the Baxter Building by Mister Fantastic that contained a spark of Zero Force. While the Fantastic Four attempted to stop him, Valeria looked up what was inside Container Zero. After the Cormorant had cracked open the lockbox and unleashed the Zero Force, Valeria arrived at the scene and flew her teleporter into the heart of the Zero Force. It reconfigured and channeled all the energy into itself, assembling a dimensional doorway to every point in space and time, which was dubbed by Doctor Doom the Forever Gate.
With the portal being unstoppable and immovable, the Fantastic Four set up a watch to guard it from any outsiders until they could dismantle it. In the meantime, Brainstorm figured out a way to operate and program the portal, creating a control for this purpose. When the Fantastic Four used the portal to bring back the Future Foundation from their cosmic journey, the interstellar beacon they employed remained active and forced the pathway to stay open. Hours later, this opening was used by aliens fleeing from the Griever at the End of All Things to reach the Earth. The Griever eventually used the portal herself, and confronted the Fantastic Four in an attempt to defeat the depowered Franklin Richards. She also intended to force Mister Fantastic to teach her how to operate the Forever Gate to use it to link up to every place and time period and fulfill her sole purpose of destruction. After Franklin cornered the Griever using the Ultimate Nullifier, Mister Fantastic came up with the idea to offer the Griever the chance to use the Forever Gate for a one-way trip into the future to the natural end of the universe so she could finish it off, and she agreed.
During the Hulk's ongoing conflict with the Leader, Jen Walters joined forces with him. After she entered in contact with The Thing, the Fantastic Four lent the Hulk use of the Forever Gate so he could travel to the Below-Place.
The Forever Gate serves as a link to every single location ever across space and time. Originally, if one were to step through the portal, their destination would be completely random. Valeria Richards managed to create a control panel that can sort through the Forever Gate's infinite variables to pinpoint any one specific location in all of time and space. The Forever Gate was also originally perpetual. With safeties of Bentley-23's design, Valeria managed to limit the functionality of the Forever Gate so only the Fantastic Four could open and close the aperture three times a month.