GameFAQs.com has a forum for the PS2 version with relatively active discussions about the game (there's also a GC and XBox forum, but they are less active): https://gamefaqs.gamespot.com/boards/561333-gladius

Speeddemosarchive Wiki has some Speedrun notes: https://kb.speeddemosarchive.com/Gladius

The Speeddemosarchive Forum has the oldest speedrun notes that I could find: https://forum.speeddemosarchive.com/post/gladius_xbox_ps2_gamecube.html