Very long process, which is why I find it to be a negative interview experience. They drag it out over a month which really piles on the stress. There is a good reason- Gartner's company culture is amazing and they want to protect that by bringing in the right people.
The short of it is that I interviewed over the phone four times, all 45 minutes to an hour, (in a more formal setting- I had various phone calls with my recruiter in between) and had one in person interview.
The first phone call was with HR and seemed to be a basic vetting process-making sure you are actually interested/qualified for the job in the most basic sense. Then I had a phone interview with the recruiter. He then suggested I do a call with a person currently in the position I was applying for. Then, I had another phone interview with hiring managers.
Finally, I was invited to an on-site interview which was scheduled over two hours. You have to prepare a presentation of yourself which is nerve wracking, because it's a longer presentation. This, combined with the interviews spanning 5 weeks, is what gives it the negative experience. Personally, I think the process is a little too lengthy and they place too much stress on their candidates.