Pros
Gartner can be a good place to work, but it is what you make it. If you're an expert in an area that is in high demand and you're willing to work hard, you'll find tremendous opportunity for recognition and progression. But not every area is in high demand. Some areas require coverage and leadership, yet aren't in high demand from clients. In some cases you can produce research that drives demand. In other cases, there are factors beyond your control. In those areas, you may find your coverage constantly changing as you and your manager try to maximize your utilization. You may also find, in spite of above-average performance, you don't get the recognition or promotion of peers in other areas. It's up to you to spot this problem and do something about it or be willing to bide your time for recognition and rewards. You can take on additional responsibilities, but beware not all contributions get the same visibility and recognition.
Cons
Company culture varies between sales and research. And, at times the culture feels non-existent because most of research works remotely. On the research side, there's also a lack of diversity across all lines (ethnicity, gender, age). There seem to be efforts in place to change this, but more still needs to be done. It seems as if Gartner has been designed as a place for ex-CIOs on their way to retirement. There are no career planning resources or defined career paths for research. Your career is what you make it, but you can work for over a decade in the same role, though with changing responsibilities and coverage. Feeling rewarded at Gartner depends on what motivates you, but the company lacks the standard systems of rewards and recognition needed to continually engage and motivate mid-level employees. Seeing peers at other companies progress while your only mark of success is intrinsic is a bitter pill. If you value longevity and flexibility and are comfortable with intrinsic rewards and modest, though fair compensation, then this can work. But if you seek extrinsic rewards and career advancement, Gartner is not the place for you.