Pros
Awesome benefits, awesome opportunities, economies of scale, great reachback support, part of a large mission, outstanding reputation, lots of possibilities for professional development and career development that comes with a company this size. If you have a good PM you will be well taken care of - GDIT seems to "take care of its own" and is generally a great work/life balance place where emphasis is on working smarter, not harder.
Cons
Due to its size, the many different projects and programs tend to operate quasi-independently - a bad experience at one project doesn't necessarily reflect on GDIT as a whole, but that particular project. That being said - you can be "just a number" given the size of the company. Recent acquisitions mean that some leaders come from "body shop" companies - they seem to be more interested in winning and keeping contracts rather than their employees. Beware of cliquishness in middle management layers - a lot of random manager types floating around meddling with no clear job description.