Superpower was introduced in Generation III (Ruby, Sapphire, Emerald). Shiny Gold is based on Generation II (Gold, Silver, Crystal) and therefore does not include this move.
Machamp can learn other powerful fighting-type moves in Shiny Gold, such as:
* Cross Chop (Level 41)
* Close Combat (Level 49)
* Dynamic Punch (TM33)
If you're looking for a Machamp with Superpower, you'll need to transfer it from a Generation III or later game.