We are still searching for game pictures and screenshots for UFC Throwdown (Gamecube). We do not have any cheats, codes, or hints yet.

Win the Silver and Gold belts with all middleweight fighters.

Win the Silver belt with a character, then enter UFC mode. Select the Free Weight option, then select the character with the Silver belt. Complete UFC mode again to win the Gold belt.
Bonus moves
Accomplish one of the following tasks to unlock another move:

Complete UFC mode and get the Silver belt.
Complete UFC mode in the Open Weight class and get the Gold belt.
Complete the game in Arcade mode.
Complete the game in Tournament mode.

Successfully complete career mode with a created Sumo fighter.