Pros
Well-respected industry leader. Great to have Gartner logo on your resume. Lots of resources to help you grow as a sales professional and to help win deals. Ability to work with incredibly talented people and build your business and tech acumen. Research content is incredible. Be careful of glowing 5-star reviews written by new hires (encouraged by HR and management).
Cons
Decent pay but overall it is below average for tech sales, which is a major reason sales talent leave after 1-2 years, along with sales reps being burnt out. Much of your success here is dependent on your sales leadership and territory. Many clients have been with Gartner for several years and you will be one of many AEs who will try to upsell them. Too much attention is paid to promoting top performers into leadership roles, when the two do not equal mutual success. At the end of the day it's sales so your performance is tied directly to your ability to sell. Expectation is you hit Winner's Circle within your first two years or you're fired.