Sea Gate is the tutorial level of Sonic Heroes. This short level is played by Team Sonic in tutorial mode, but it is also the first level played by Team Rose in their story, the stage introduces new players to the controls of the game.
Using the same environments as stage 1 (Seaside Hill), Sea Gate walks the player through generic controls of a 3D Sonic game as well as demonstrating how the team play mechanic works. Power Cores, Change Gates, and basic attacks like Thunder Shoot and the Homing Attack are explained by the disembodied voice of Omochao.
There is no legal way for Team Dark or Team Chaotix to participate in this level without a memory editor or other cheat code to make these teams appear in the level.