From the Super Mario Wiki
The Gold Star is the third Crystal Star in Paper Mario: The Thousand-Year Door. It was supposedly found on the Glitz Pit Champ's Belt, but Grubba had the real one and used it to power his Strengthening Machine to turn him into Macho Grubba. The Gold Star was given to Mario by Jolene after defeating Macho Grubba and reuniting her with Prince Mush.
This Crystal Star will teach Mario the move Power Lift, which costs three SP. To use this move, the player must aim a cursor at a 3x3 grid of stars. Hitting orange arrows raises Mario's attack, blue arrows raise Mario's defense, and Poison Mushrooms slow the cursor down for a few seconds.