Sigh.. I know this................
My question is:
Should I train my EVs to max before leveling my pokemon or can I level up my pokmeon while maxing the EVs? will the stats bonus gains be the same
I always lvl my pokemon while EV training them, saves precious time.
but that's my opinion son, however there are some pokemon who are abit harder to EV train so you do it in low lvl places.
in your case Gastly is super easy to train, so I advise to do it while lvling.