Specifically the Symfony PHP framework, which is MIT licensed. I've read that the two are compatible, but I want to make sure that a system I develop can be released complete under the GPL.
Yes. It is possible to release a project that uses MIT licensed libraries under a GPL license. The MIT license is a permissive license which has
no very few restrictions on code that links to its binaries.
Incidentally, you could also include actual MIT licensed source code in a GPL project. It's very developer friendly that way.
However, this is a one way compatability. You cannot do this the other way around and include GPL code in an MIT project because the GPL requires code that links to it also be licensed GPL. Once you let any GPL code or libraries into your project, you must license the whole project under the GPL.