Where are you in the game? If it is before you get the national dex upgrade, that is the reason. You get that message when a pokemon tries to evolve into a Gen 2 pokemon or later. Just keep playing and eventually you'll get it, and then you can evolve your Gloom.
Oh! Thank you so much for answering so fast!
I'm at the beginning still, actually. I'm the type to grind my pokes for hours on ends, haha. That's why I have such high levels so early.