Guile is Free Software. Guile is copyrighted, not public domain, and there are restrictions on its distribution or redistribution, but these restrictions are designed to permit everything a cooperating person would want to do.
C code linking to the Guile library is subject to terms of that library. Basically such code may be published on any terms, provided users can re-link against a new or modified version of Guile.
C code linking to the Guile readline module is subject to the terms of that module. Basically such code must be published on Free terms.
Scheme level code written to be run by Guile (but not derived from Guile itself) is not restricted in any way, and may be published on any terms. We encourage authors to publish on Free terms.
You must be aware there is no warranty whatsoever for Guile. This is described in full in the licenses.