From Sonic Retro
This character is never called "Angelus the Gatekeeper" in the Japanese version of the game, or any version of the game. The name isn't used on the website or the guide book. It appears to be a fan name. Sonic Riders Saikyou Kouryaku Guide calls him 守護兵, which translates to "Guardian". The mission goal before fighting him in the English language version of the game is "Beat the Guardian!".
Images from the guide book: https://imgur.com/a/26N4e1u