I don't mean to contradict anyone, and very well know many people can and do experience problems related to use of nail polish, gels and acrylics. However, all gel-related damage I myself have seen has been mechanical—meaning that it has occurred from lifting and/or non-gentle removal. My own nails have always stayed quite healthy as long as I've maintained them properly. That also means maintaining proper hygiene.
While normal manicure is removed at least once a week, gels can be kept for months on end. Due to this, any lifting should be taken care of as soon as possible not only due to potential physical damage, but also because you can't really clean the space under a lifted section. If anyone close to you has a fungal infection, a nook like that can help it colonize. That's why I also wouldn't recommend using gels on your toenails if you have athlete's foot or live with someone who does.
As Julia said, it's really best to go to a salon unless you really want to make an effort to acquire the skill. Gel manufacturers do offer courses that you can attend, though, if you do want to learn. The prep work is what matters the most, and consists mostly of what you'd do for a really proper traditional manicure.