The elves were brought to Gielinor from their ancestral homeland through the World Gate by Seren. The elves arrived in Gielinor during the First Age. According to The Crystal of Seren the gate was sealed along with the undercity of Prifddinas during the second age when Seren departed from Gielinor. This cut off elven access to Crystal, their primary resource. It is currently unknown whether the World Gate can be reopened by any means other than the intervention of a god, or any god other than Seren. How it was "sealed" is vaguely described at best. Physical access could have been cut off or it could be magically disabled.
Elder Mage Etheron of the Mage Conclave of Kethsi spent six years in the outer planes on an expedition. He returned through the World Gate with the Stone of Jas, which allowed the Kethsians to cast spells to better their civilisation. However, the stone later brought the Dragonkin, which ultimately caused the demise and ruination of Kethsi.
The World Gate was in Isafdar at the time of the elves arrival, and due to its consistent link to Seren's Ancestral Home for nearly two ages likely remained there. One theory of its exact location is that the gate is in the undercity of Prifddinas which was also sealed when Seren left Gielinor during the Second Age. This is supported by the logic that the elves would build their crystalline capital near their access point to crystal.