Pros
As far as CSM roles go, this one is pretty good. Pay is good, although not as good as in SaaS - then again, you aren't worked nearly as hard as a tech CSM. If you have good organizational skills and are very comfortable saying no, excellent work life balance is easily attainable (if not, it will be awful). The day to day job is varied and interesting enough, and it is gratifying to build long lasting client relationships. Great C-level exposure and opportunity to build executive presence. Gartner has an extremely generous PTO policy, and in CS you are actively encouraged to use it all without policing of timing (not the case in sales). Colleagues are generally smart and pleasant. Culture is generally accepting and inclusive. Overall, there's a lot to like about the company.
Cons
"Personal brand" is the key to your success at Gartner. It is discussed constantly and seems to be the main driver for promotions. In CS, personal brand is how much sales/their leadership respect you; if they think you are incompetent and/or a pushover, they can and will make life unpleasant. The product does not inspire a ton of confidence; the value of research/advisory is very difficult to quantify and it seems like more of a nice-to-have for a lot of clients. Gartner operates in a funny space between consulting and typical research firms; it's a lot cheaper than consultants but we don't do the work for clients, while it's extremely pricey for "insights" alone.