I had no idea there was work being done to implement shared memory coarrays in gfortran! This is such lovely news, however preliminary or unstable it may be at the time I’m writing. You can read about it in this thread (which continues into December) https://gcc.gnu.org/pipermail/fortran/2020-November/055370.html
Three cheers each to Nicolas, Dominique, and Thomas for undertaking this effort, and any others I may have missed in my excitement!