Doubtful that it will show any black under the operculum at that size. (Edit: Just read it was 4", probably would)
loves severums - that gravel is no where near small enough. Ideally they should be kept with sand fine enough to be sifted in through the mouth and out the gills.
Can you see that gravel even fitting into it's mouth, let alone coming out of it's gills? Absolutely no chance.
Once again, how big is the tank? I'm going to go ahead and recommend you return the fish anyway.
G. abalios from what I can tell. G. brachybranchus has a bar on their cheek and I can't see one from the photos.
I hope this is a bigger tank than what you've had in the past. I agree with Japes, you are going to need a bigger tank than the 55g you've posted about in the past.