To broaden the discussion one thing I like to do with creatures like gargoyles is never identify them as what they are. Describe them as statues coming to life and even if the players guess gargoyle I just slyly answer maybe. If the player complains about the detect magic I would tell him something like "I guess you do find it weird that statutes coming to life were not magically. MAybe you want to investigate it further." or something that can potentially turn it into a plot point and a little mystery.
Thiiiis. The only time I play it straight is if I have new or inexperienced players. Otherwise, nothing I put on the table is by the book. None of the monsters have UPC symbols on them, and they don't come with return receipts. I certainly don't go naming the monsters for them, unless they're very familiar, and the monster lends itself to that sort of familiarity. The only time players can expect to know how a monster works behind the screen is when I want it that way. In fact, my general assumption for any given monster is that it's unique (until it isn't), unless it's something that obviously isn't, like orcs or halflings. Definitely for the exotic monsters that don't get much use. Playing by the book seems flat when you're dealing with players who have the MM memorized. Not that you have to be constrained in this direction, either, because a card-counting player can't count cards with confidence if you throw him for a loop often enough.
Nope, you weren't in the wrong.
Forgot to mention, I reject the premise of the question, if that isn't already obvious.

The gargoyle radiates magic while in "hibernation", or doesn't, depending on how you want to run your game. The gargyle radiates magic after leaving "hibernation," or doesn't, depending on how you want to run your game. Maybe your world's just different, maybe they gargoyles are just different, maybe the dungeon is enchanted, maybe a wizard cast a spell on the gargoyles, maybe the warlock is under a curse...ad infinitum. Personally, I actively resist being pigeonholed, as your player is trying to do to you. Reading the Monster Manual doesn't make a player the DM for your setting. That's the beauty of a real RPG - the only limits are the imagination.
We used to call these people "rules lawyers," back in the day. Has that epithet lost its sting these days?
PS, new DMs: consider renaming every monster or magic item you pull from a book and use in your setting, particularly official, 1st party books. You can use ideas from a
good translator, or roll your own. They don't have to be great, they just have to make them
yours...not Wizards', not the books', not some rules-lawyer's.