Where can you find a gabite in Pokemon Diamond?

you can find some of them in victory road. but you need national dex as if because in this area, you will team up with a trainer named Marley and she has an Arcanine (this Pokemon is national and he is out of the Sinnoh Dex)

the easier way, go to Route 207 then go near the entrance of Wayward Cave. then, go under the Bicycle Road then go to the grass patch on the other side, then go 2 steps to the right, then keep going up. there is a secret cave. inside, there is a staircase going to the basement. this is where you find Gible. catch one, grind him up to level 28, then he will evolve to Gabite.

and you want a Gabite i know!