When you first defeat Gar'skuther, you will receive his legs. Choose Replay mode and select 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 afford them easily 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.