Assuming you mean the Pokemon Gastly and not some other Pokemon unknown to me (of which there are few), you can find it in the Old Chateau near the exit to Eterna Forset. You will find that it is the only Pokemon in there, other than Rotom, which I suggest you don't bother getting coz it's stupid.
Trade a Graveler in Jubilife City in the Global Trading Center and it will involve after the trade.
The first place you can find Ghastly is in the Sprout Tower located in Violet City.
you do not find dialga in Pokemon pearl
use gastley as a ghost Pokemon and us its poke assist
Trade with Pokemon Pearl or Pokemon Platinum
the best Pokemon you can catch in Pokemon pearl is heatran
go to the sprout tower in the town where you get the first badge at night.
You don't find a Krookodile in Pokemon Pearl
where do i find a totodile in Pokemon diamond/pearl on nintendods
you cant find raiku in Pokemon pearl just Pokemon gold
You find Gligar in Pokemon Platinum at Route 206. You can't FIND IT IN PEARL.
You can go on gamespot.co.UK and search for Pokemon pearl