Behavioural styled interview, that is intense and requires a high amount of self reflection and dedication. Due to COVID mine was online.
First stage is a one hour interview with a Gartner recruiter.
Second stage is similar to the first interview, but it is important that you take on the feedback from the first interview and improve for the second. This demonstrates your willingness to learn.
Final stage is a panel style interview with VP's (usually 2). This is a fairly long interview, where the first part is behavioural style questions and the second is how you can understand and explain information they provide to you.
Tips: This is tough, but in my personal opinion that is a good sign. It shows that Gartner will push you to be the best version of yourself. They encourage growth and development and their interview process really demonstrates that. Having done many other interviews, Gartner was definitely the most difficult, but was the one I learnt the most from. Take on board their advice, and even if you don't get the job you will gain something out of the process.