Yes, the ROCm stack and associated kernel drivers are open source.
https://rocmdocs.amd.com/en/latest/Installation_Guide/Software-Stack-for-AMD-GPU.html
If you scroll down to the "Drivers, Toolchains, Libraries and Source Code" section you will see a list of component names, with each component name linking to the corresponding source code repo on Github.
We also maintain a full set of open source graphics drivers, upstream in the Linux kernel and various freedesktop.org (fdo) repositories.
Note that the ROCm stack is currently being released for Linux only. A Windows version is in development, however the Windows version will probably not be fully open source.