Pros
Not many. I work as a contractor and pretty much every pro is on the client side of things, not CGI's side.
Cons
CGI forces you to get health insurance with them, doesn't pay for all of it, and makes you choose between plans that take up most of your paycheck, or plans that have $3,000 deductibles, something I would never meet in a typical year, meaning the basically pays for nothing. They make you use Paid Time Off even if you are told not to come to work for weather related reasons, and this year for Juneteenth, even though it was a federal holiday and the entire facility was closed, they made us use PTO for that, too. They give you zero sick time, and if you need sick time off you either use PTO or Leave Without Pay, which they only give you a small amount of, and if you use all of that in a year you get fired. They pay very low wages and sometimes it's up to 6 years before you get any kind of raise at all. They force you to do overtime on weekends even though for direct employees of the client, overtime is optional. All this is just the tip of the iceberg. They just generally show a blatant disregard for contractor employees, treating them as mere resources to be exploited.