While Linux-libre is the default for Guix there are no limitations in place that would keep you from using vanilla Linux. In fact, Guix makes it extremely easy

While Linux-libre is the default for Guix there are no limitations in place that would keep you from using vanilla Linux. In fact, Guix makes it extremely easy to build custom packages, and that includes custom kernel packages. You can augment the package collection that comes with Guix with a simple environment variable, so the insistence on software libre on the side of the project should not represent a technical hurdle.