I’ve never heard of it but it does look cool. This quote is off their terrible website and sums up why it’s not part of the roadmap for the Librem 5.
At this stage, GNUnet remains an experimental system where significant parts of the software lack a reasonable degree of professionalism in its implementation. Furthermore, we are aware of a significant number of existing bugs and critical design flaws, as some unfortunate early design decisions remain to be rectified. There are still known open problems; GNUnet remains an active research project.
Welcome to GNUnet’s documentation! — GNUnet documentation