It's a little black and white to say that it can't be a lifestyle. But it's sort of the difference between "gamers" and "people who play games." The majority of the gaming market falls into the second group - they enjoy games, maybe they wear the occasional T-shirt, but it really sort of ends there. Yes, it would be a hobby for these people.
But as for the first group - not only do we typically play games more than the average "person who plays games," we play them in a different way. I call back to MovieBob's "A Nerd By Any Other Name" episode of The Big Picture - it's not just what you're into, it's how you're into it. Some of us take it quite seriously, we play the games, some of us actually make them, we wear the T-shirts, buy the collectibles, go to conventions, associate with others who share our interests, we have our own slang, our own terminology, our own inside jokes and references, our own magazines, websites, news columns, radio shows, our own entire culture. We live our lives that way. And in that regard, yes, gaming can absolutely be a lifestyle.