Let me first say that I did not go to Gnomon, however, I do know people there, and I do have a pretty good understanding of what they're all about. Alright, with that out of the way...
To answer your question, Gnomon is not necessarily for people already in the industry, although they do quite a nice job of training employees for companies. The complete course, which is 4 terms, only accepts 8 students per year (last time I checked). To get in, you have to have a great portfolio and then follow that up with a great interview. When I say "portfolio", it doesn't have to be 3D work, any type of art at all. Whatever it is you love to do. They maintain 100% placement, which is easy considering a school of that calibre is only putting out 32 students a year, so they are in demand.
The workload will be tough, and it's best if you have some kind of idea what you want to do before you just rush in there. The people you'll work with there, the instructors and the students next to you are exceptionally talented people, no question about it. You'll be reviewed by people in the industry constantly, and you'll get a chance to network better than anywhere else can offer you. I mean, it is after all, Hollywood. Give them a call, they'll send you a brochure, and ask around. They've got a great rep as far as I know.
If any of my information is wrong, I ask that anyone who is more familiar with Gnomon would post and correct me. Thanks.