Also, in some circles, meaning Gender/Sexual Minority - to some a better catchall term than gay, LGBT or QUILTBAG. The previous acronyms make every attempt to describe what people are by iterating through the various possibilities - and include them all.
In the case of human sexuality, homo/heterosexuality describes in essence a binary, with a bisexual as some kind of fader switch between the two poles. Given that gender is fluid, and there are most certainly more than two, sometimes encapsulating difference in binary language is futile.
In the case of gender, more so. We think of people as cis-male, cis-female, trans-male, trans-female, but in the case of intersex people and the whole notion of gender construct, there's really no point in seeing that as a polar binary either.
Understanding that anyone off the hetero- axis and off the cis- axis is going to be a minority, the term GSM does a lovely job of describing a possibility without enclosing it in a box, and leaving the person in such a sphere to continue without having to be stuck into or judged based on the label on the boxes.