Welcome to GCCMVS (GCC for MVS and CMS)
GCCMVS is a standalone compiler designed to run on MVS or CMS. It consists of two distinct parts. Firstly, the standard GCC 3.2.3 compiler with some required code changes for MVS and CMS. Secondly, PDPCLIB, which is a C runtime library require for GCC to be able to run, and also for your own programs to be able to run. Both things are required, and both things are what is dubbed "GCCMVS". Note that the licence for the first part is GPL while the second part is public domain. The second part being public domain means that there are no licencing restrictions on executables you produce yourself.
The source code and executables for GCCMVS can be found at sourceforge, in the "downloads" link. There is also a port of GCCMVS to MUSIC/SP which is available here.
There are a few introductory tutorials for using GCCMVS on MVS 3.8. They are currently in the process of being updated to GCCMVS 5.0, but the content is still relevant.