So, suppose my agent tells me that a publisher is looking for easy readers with strong hooks, like "pirate mom" or "dancing dinosaurs." What's the difference between a hook and a gimmick?
A hook is the reason a reader decides to buy (or read) a book.
A gimmick is an attention-getting element that does little more than get attention. It is a contrivance that seems to add value without actually adding value. It distracts the consumer from the question of whether she really wants the product in her hands.
So while there are many, many hooks that are not gimmicks, all gimmicks are meant to be a form of hook.
I don't think I agree that the subject of a book can be a gimmick. It's the subject. If you aren't interested in buying a book about pirate moms or ballet dinosaurs, you aren't going to be distracted from that lack of interest by the fact that the book is about pirate moms or ballet dinosaurs.
What your agent (and the publisher) want to remind you of is that the goal of any early reader is to be appealing enough to get a child (for whom reading is difficult) to read it. That means it needs a hook that is not
1. great writing (because they can't discern great writing yet)
2. sparkly covers or novelty elements (because early readers are low budget) or
3. name recognition (because they do not recognize author names yet)
...and that leaves you with topic. You have to give them topics they know they want: dinosaurs, sports, ballet, pirates, superheroes, goofy jokes and puns, etc etc.