The Realmgate is a part of 's endgame, giving those who find it access to pinnacle bosses in the game. These foes are the strongest opponents you could possibly face, giving anyone with endgame builds the challenge of a lifetime.
However, you can't use the Realmgate until you understand where it shows up and how it functions. Normally, map nodes are accessible through Waystones in once you reach the endgame content. The Realmgate does not use Waystones though, as it has specific requirements you have to fulfill before you can face pinnacle bosses.
Do not worry about farming Waystones to activate the Realmgate; simply see what its requirements are to start battles. Once you beat the main game, you should find the Realmgate on the endgame Atlus map.
The Realmgate will appear close to a starting point on this new map, which has several nodes leading to many familiar locations.
Most of the time, the Realmgate shows up close to the Zigurrat Refuge, an area to the south where you begin your endgame adventures.