Oh, finally. There's plenty of GNSS/GIS folks lurking around the OpenStreetMap wiki and I've heard their telegram too but never explored it. There's some GNSS-but-not-GIS folks on the Time-Nuts mailing list. Galmon has its own little handful. There doesn't seem to be a good RTKLIB forum, which is weird.
I hope this takes off and unites a whole bunch of little communities.
You may be interested in joining the Spatial Community slack if you haven't already. There's a self-invite link at https://thespatialcommunity.org. There's a ton of channels there spanning a range of GIS related interests.
I hope this takes off and unites a whole bunch of little communities.