My life would suck without you
Does nature increase the positive one(like EVGastly -> +spc.atk) on level-up? Or is it like a constant value
Nature increases the positive stat by 10% and decreases the negative by 10%.
Unfortunately your shiny Gastly has very low IVs and is comparatively weak. Your other Gastly has the chance of being flawless in SP Attack however.
You need to have Dive and the 7th badge, the dive and find the room with the submarine. Team Magma will be there, defeat them and Groudon will go to Sootopolis City. Catch it or defeat it (you should totally catch it though) and then you can get the 8th badge, go and beat the Champion. And be a happy birdy.