The Gungan Round-up level is accessed by finding the three Gungan artifacts throughout the game. They are in the following locations:

Naboo Swamp
Near the end of the second level down in crevice.

Tatooine
On a medium size plateau up against the screen. Jump forward in a leap of faith. It is inside the hut with the Jawa in front of it.

Gungan Ruins
Near the end after the covered trap, right next to a tree and a large patch of bushes.