User blog comment:FairyRaven/Fairy Tail Chapter 408: The Definitive Demon, Review and Commentary/@comment-24277053-20141110135803/@comment-25111584-20141111084232

I can come up with quite a few explanations for the Gray thing, from him being able to sense Demons due to his power (Jiemma was heavily implied to have been hiding his presence) to something as simple as Silver having given him a lot of info about Tartarus. Actually, the same goes for most of the plotholes people were talking about in the recent anime episode (though it was still pathetic >.>), so not bothered with any of these. I wouldn't mind if Mashima didn't explain it either, it's pretty trivial and as long as I can come up with a satisfactory explanation for it, I have no problems with trivial things being left unexplained.

Tbh, I actually thought that it was somebody else speaking the whole time while just Gray's face was shown in the panel to give everybody's reaction to Jiemma, but, looking at all the comments, I guess I was wrong ... not to mention Natsu shouldn't know either and Sting and Rogue don't call her Minerva. >.>

@Nick: Considering Gray's proficiency with the new Magic and considerable power, I always thought that Silver had transferred his knowledge of the Magic, Tartarus as well as his Magical Power to Gray. So I never really bothered with his OPness or expertise with the new Magic.

As for perfect proficiency, though I'd say Silver passed his knowledge along, doesn't Mashima show that the characters can use powerups perfectly right off the bat?
 * I think the only one who had this done well was Mira, who was actually ostracized for her Magic and then had to train to gain control of it.
 * I guess I'd put Gray's IDSM (not BDSM, FTGM) and Natsu's LFDM on the same level; at least the two know how to use Magic. And Natsu just got another element to a Magic he already knew how to use while Gray mp got Silver's knowledge of the Magic, not to mention the element is the same (though, imo, the element part doesn't do much in Gray's case, it was just an all-too-convenient-plot-device).
 * Lowest, I'd have to put Erza's Magic. Here's a girl who is implied to have absolutely no experience with Magic, but she suddenly goes, "Kyaaaaa!" and gets a new Magic with which she absolutely annihilates the ToH's whole Magic Corps. Having a load of Magic Power like Ultear is one thing, but even she had to learn Magic later (I'm assuming she learnt Ice-Make, no matter how much she hated it. :P). Erza not only just got her Magic, but appeared to be extremely proficient with it, being able to target only enemies, controlling a load of weapons, not injuring any friends (yeah, I know they were all behind her, still a far stretch), etc.

Still, Mages being able to use a new Magic perfectly right after gaining it is not a new thing. It's not a good thing, but one slowly becomes immune to this and doesn't think much about it, just like I have for Erza's battles, to a degree.