Successfully complete the game to be able to replay the levels with everything already collected. You can now try to collect everything if any items were previously missed.
Purchase the indicated item at the Cheat Shop to gain the corresponding cheat. Note: Some items cannot be purchased until the game has been completed.
When you first defeat Gar'skuther, you will receive his legs. Select Replay mode, and choose the planet you were first on. Then, go to the Sporepedia, and select "Badges". Go to Park Shop. When you go down, you will see the other parts. Purchase them. You can easily afford them when you complete the game. Go to the Creature Creator. Add the body, and remove the other parts to create Gar'skuther. He may be one of the weakest and easiest to get, but he has a lot of abilities that work nicely.
Defeat Gar'Skuther, then replay the Tapti planet. There is an island you can reach with the water walk ability. Go to Fryse, which is the first region. Near the place where you exit into the cave to go to the next continent, look in the water on the top screen map. Notice that the water is dark blue and crossable. Go southeast until you get to an island. On this island should be a Skuther, two Oogies, and a pollinated creature. These two Oogies are from the other eggs that yours was with at the beginning.