The interview process took longer than I expected (5 months) with multiple "blackout" periods during which I convinced myself that Gartner was no longer interested in me. I'm posting here for anyone who might be in the same boat -- don't give up! It turns out I was a unanimous decision (which seldom happens), but my perception throughout the process was that they were ambivalent. It turns out, they were just really busy. Gartner is growing and hiring like mad. They just don't have time to keep candidates posted on the details of the application process.
Others have written on the interview process (multiple phone interviews with research VPs and a final with the managing VP; in-person interview includes a 90-minute writing exercise, and then Q&A from the full research team on what you've written). The only thing I might add to prep is to get comfortable with talking to multiple people on Skype and with a Thinkpad/Windows machine (I'm a Mac person).