Alright, my previous review was overly angry. I'm going to write a new one...
First of all, were the writers seriously thinking that being overly oblivious about your pet being in danger is funny? Because, it's not. It's irritating. No, scratch that, it's more than irritating. Enough said.
Second of all, SpongeBob stole the freaking thing! That is wrong. Behinds, he completely ignored the pet shop owner's warnings. Already, you know something bad's going to happen.
Third of all, SpongeBob has left Gary alone multiple times before this episode, yet he doesn't start worrying about that until NOW? What the heck, writers? There's bad continuity, and then there's obvious plot holes.
Animal abuse seems to be lit in the "funny" light in this episode. It's not funny. It's sickening. And this episode only sledgehammers that in even further.
I think I explained very well why this is one of the most hated episodes of the show.