You can't, unfortunately. :c Gallade was made in the 4th generation of Pokemon, and Pokemon XD: Gale of Darkness was set to take place in the 3rd generation.
Sinnoh Pokemon like Gallade, Electivire, Magmortar, and Pachirisu were in the fourth generation. The only games where you could get these Pokemon (and many other Sinnoh Pokemon) are Pokemon Diamond, Pokemon Pearl, and Pokemon Platinum.
Kanto, Johto, and Hoenn Pokemon were included in Pokemon XD, as well as many other games (not all together; that was only in Colosseum. Separately, as in each region/generation had its own game).
If you want Pokemon info like how to get what, then go to Serebii.net. It's a very good source. (By the way, to get a Gallade in Diamond, Pearl, and Platinum, you need to use a Dawn Stone on a male Kirlia.) C:
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u need a male ralts u can get it from route 209 th en u need to evolve it into a krilla and u need a dawn stone and ur krilla will evolve into a gallade or u can get a male krilla on route 209 and use a dawn stone on it it will become a gllade
you can't catch a wild gallade , only by evolving a male gardivoir
Sorry to bust your chops but it does not work like that. Actually, You gotta give a Male Kirlia a Dawn Stone and it'll evolve to Gallade.
I'd say Garchomp, Gallade, Magnezone, Electivire, Magmortar, or Mamoswine.
There's no Pokenav since Pokemon Light Platinum's build is from Pokemon Ruby.
Kirlia evolves in lvl 30 in a Gardevoir or when it is a male and you use a Dawn Stone it evolves in a Gallade.