Garchomp can only learn Outrage if you breed it.
Go to the battle frontier
By breeding.
Breeding outrahe is an egg move
Salamence can't naturally learn Outrage in Diamond, Pearl and Platinum and it can't learn it through breeding either however in Platinum you can teach it Outrage by going to the Move Tutor in the Survival Area located in Route 226, you need Rock Climb to get up to the house as well as 6 Red Scales and 2 Yellow Scales in your bag in order for the Tutor to teach Salamence the Outrage move.
Garchomp can only learn Outrage if you breed it.
Go to the battle frontier
By breeding.
Garchomp doesn`t learn Outrage! XXX A Girly
Breeding outrahe is an egg move
Salamence can't naturally learn Outrage in Diamond, Pearl and Platinum and it can't learn it through breeding either however in Platinum you can teach it Outrage by going to the Move Tutor in the Survival Area located in Route 226, you need Rock Climb to get up to the house as well as 6 Red Scales and 2 Yellow Scales in your bag in order for the Tutor to teach Salamence the Outrage move.
Outrage, Stone Edge, Earthquake, and Crunch.
The 111th Pokemon on Platinum is Garchomp.
There is a move tutor in the Frontier Access who will teach ampharos Outrage for 48BP
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.
in batman city
A good moveset for Garchomp is: Outrage/ Earthquake/ Brick Break/ Fire Fang. If you want to, you can swap fire fang for crunch or stone edge.