Well, the Oregon State University Open Source Lab is now hosting the GFortran Mattermost instance: https://gfortran-mm.osuosl.org/
Those people made a great job, importing the whole content of the previous instance. One thousand thanks! 
As stated above by @JerryD :
If anyone here would be interested in joining the gfortran developers team, send an email to gfortran@gcc.gnu.org or myself at jvdelisle@gcc.gnu.org.