Gartner has an extremely grueling interview process (and they should as you are expected to guide CIOs and other decision makers). The process starts with an application but I'd highly recommend you get involved on some sort of social media first and get to know some analysts first, this way you have someone that can put your resume at the top of the pile.
You will get 2-4 phone interviews, a couple will be technical and I don't care how smart you are, you will be stumped. This is done on purpose to see how you handle yourself (again this will happen throughout your job as well). If you get past this process you will be brought in for a face to face interview.
Before you have the face to face interview your future manager will send you a topic they would like you to write a report on. You typically have one week to write this paper. This is to test your writing style and if you can deliver on a deadline. Be warned though different departments handle this differently, some departments will have you come to the face to face. Give you a laptop with no Internet access and a topic and ask for a 2-4 page paper, and leave you in the room for 2 hours. Once your 2-hours is up you will get a 30-minute break and if you return from this break (some don't) you will then being the face to face team interview.
The team interview will be over a video conference. They will make you defend your paper you just wrote and most likely you will be shredded to pieces in this interview too (this is by design). Again, the idea here isn't to see how well you write in 2-hours under pressure with no access to the Internet, the idea is to see what's bottled up in that brain of yours and to see how well you respond when you don't have all the answers. A few things to understand about the team interview, once you leave, every member of the team has a say in if they recommend they hire you or not. You won't win them all try to win most.
It sounds worse than it is, just remember to relax. Also remember that the interview is the job, if you hate the interview, you may hate the job.