It's the kind of thing that's hard to nail down, but it's generally accepted that about a third of teen suicides are LGBT kids. A lot of the information I've found online cites studies that are twenty or thirty years old, though, and I'd like to see some more recent data.
In 2007, the last year for which final data are available, 184 kids between the ages of 5 and 14, and 4,140 people between the ages of 15 and 24 committed suicide in the U.S. (
National Vital Statistics Report - Deaths: Final Data for 2007.) That's all suicides for those age groups, not just the gay kids.
Family reactions matter a lot. One 2009 study (
here) found that "lesbian, gay, and bisexual young adults who reported higher levels of family rejection during adolescence were 8.4 times more likely to report having attempted suicide, 5.9 times more likely to report high levels of depression, 3.4 times more likely to use illegal drugs, and 3.4 times more likely to report having engaged in unprotected sexual intercourse compared with peers from families that reported no or low levels of family rejection."