You can't. You need to evolve Gabite.
The 111th Pokemon on Platinum is Garchomp.
Garchomp cannot learn Giga Impact from leveling up in Pokemon Platinum. Garchomp can learn Giga Impact from the TM that teaches it which is TM68.
Cynthia elite 4
in batman city
What you do is, you got to any Pokemon center. Got to the second floor and trade with a friend that has it in diamond, pearl, or platinum.
when you battle Pokemon champion Cynthia in Pokemon pearl,platinum,and diamond her strongest Pokemon is garchomp
Cynthia has: Spiritomb Lucario Garchomp Togekiss Milotic roserade
Level 78
She has six.Spritomb,Togekiss,Milotic,Garchomp,Lucario and Roserade.
diamond and pearl spirttomb garchomp roserade gastrodon milotic and lucario platinum spirttomb garchomp roserade lucario milotic and togekiss
The champion is Pokemon Diamond, Pearl and Platinum, is Cynthia. She has various Pokemon-types. She is the Sinnoh Champion. Her Pokemon are Spiritomb, Lucario, Togekiss, Milotic, Garchomp and Roserade. In Platinum, Cynthia has a Togekiss. In Diamond and Pearl, she has a Gastrodon.
catch a gabite on victory road and level it up to lv48 good luck :)