The first real fantasy I read was Narnia. After that, I was a fantasy nut. Everyone has to start somewhere, after all. Basically any book can be a "gateway book" if it's the first someone has read in a particular genre. On the other hand, I've given up completely on some genres after reading one book. So it's a double-edged sword. It might make them want to read more, and it might not. Someone with good taste in other genres might judge fantasy harshly based on Eragon, where if they read a better, more mature book, they might get sucked in. It's all a crapshoot really.
As a parallel example, take a look at the anime Dragonball(or pokemon if you're more familiar with that. Same effect really). It was a huge gateway show for the anime genre/style of television. Kids liked it, and they moved on to like anime that most "real" fans consider to be high quality stuff. Other people found it hopelessly inane, and missed out on shows that are really quite good. I moved out of the DB novelty phase, and stopped watching anime for six years. If I hadn't happened on another anime show--which did lead to a continuing interest--while up late for no reason, I wouldn't be an anime fan, and I would have missed out on dozens of what I see as reasonably high quality shows. DB would have turned me off forever, after the novelty was gone.
The same thing could happen to the fantasy genre after a kid reads Eragon. On the other hand, plenty of people developed a continuing interest in anime after seeing DB, and I'm sure Eragon has had the same effect for many people in terms of fantasy. I might or might not have been one of them. It's pretty much random. You can't predict how it will turn out, or apply generalizations.