You have to use the Poketransfer and bring it over from a previous generation.
Gastrodon is a Water and Ground type pokemon.
Garchomp and Gastrodon
Gastrodon
If you go to the pokedex, select Gastrodon and touch AREA, it will show you.
You can either catch a Shellos or you can find a Gastrodon in water areas I think.
Gastrodon
Gastrodon is #423 in the national pokedex, and it is a Water-Ground type Pokemon.
Nidoking and Jolteon
gastrodon is on faldera volcano in a chamber next to the camber that leads to the path to where moltres sleeps
Level 28
turtwiggrotletorterrachimcharmonfernoinfernapepiplupprinplupempoleonstarlystaraviastaraptorbidoofbibarelkricketotkricketuneshinxluxioluxrayAbrakadabraalakazammagikarpgyaradosbudewRoseliaroseradezubatgolbatcrobatgeoududegravellergolemonixsteelixsheildonbastiodonmachopmachokemachamppsyduckgolduckburmywormadammothimwrumplesilcoonbeautiflycascoondustoxcombeevespiquenbuizelfloatzelcherubicherrimshellos (west side)shellos (east side)gastrodon (west side)gastrodon (east side)heracrossaipomambipomdrifbloomdrifblimbunearylopunnygastlyhauntergengarmisdreavousmismagiousmurkrowhonchkrowglameowpuruglygoldeenseakingbarboachwhiscashchinglingchimechostunkyskuntankmedititemedichambronzorbronzongpontyarapidashbonslysudowooodomime jr.mr.mimehappinychanseyblisseycleffaclefairyclefablechatotpichupikachuraichuhoothootnoctowlspiritombgiblegabitegarchompmunchlaxsnorlaxunownriolulucarioif you want to know all of the Pokemon and check them out, go to internet and type in pokedex and press enter. wait for a few seconds and click the first site you see.
You can't see it wild on Pokemon White but you can see it in Route 10, Route 11, Victory Road and Village Bridge on Pokemon White.