my take on it is that gc is better at running long-lived projects or projects with non-developer users. github is great, but its emphasis is strictly on coding and dvcs.

i've run svn gc projects, and projects on github, and found the gc stack to be generally more robust, cohesive, flexible, and methodically designed. ex: compare the rich functionality of the gc issue tracker to github and you'll find them incomparable. or, create a project and browse the "advanced" tab of your project and you'll probably find you have more control than it feels like, with little effort.

regarding "can actually host content" - prolly a red herring because both let you host content. they just present a different ui for it.