Distributions can have licenses. Most modern distributions are MIT licensed (meaning you can create derivative distributions under the conditions of the license). OpenSUSE is still GPLv2 and is one of the last ones (Debian switched to MIT recently IIRC).
But most of that software will be licensed under other licenses, and you can't just redistribute it under another license!
If you ship a collection of software called a "distribution" that includes a copy of the Linux kernel, then that copy of the kernel remains GPL. The fact that it's on the same CD or server as some other MIT-licensed software is irrelevant.
A collective work is separate from a derived work. These are two separate things in copyright law. You can own the copyright over a library of music, despite not owning the copyright over the actual music itself. So, you could have a proprietary Linux distribution that is only made up of GPL'd software. The only thing that would be "proprietary" is the particular configuration, selection and build scripts of the packages that you picked.
So, having a GPL'd distribution means that the package sources and configurations and other such "distribution sources" have to follow the rules of the GPL. It doesn't matter what the license of the software itself is (it can even be GPL-incompatible or proprietary).
