Even the strictly GNU parts are in the minority in modern Linux usage. There are dozens of libraries, some GPL and some X11/zlib/Apache licensed but most of them not part of the "GNU project". Some parts only run on Linux and Linux-compatible systems because they were built on top of Systemd or other Linux-only software.
FreeBSD has had a Linux emulation layer for quite a long time, so in that sense you can of course have a Linux environment without a Linux kernel, just as you can have (with WINE or ReactOS) a Windows environment without the Windows kernel.