This is pretty lame and I’m not sure how it happened but that group being in only /etc/gshadow means normal users can’t read the file. The group definitions you want to use need to be in /etc/group. There are numerous groups in gshadow and not in group by default on silverblue. Not sure why yet. You can get everything working like this:
getent group dialout
This gave me the group and the id in the proper format your group id may be different
dialout:x:18
I then wrote the line with my user to the /etc/group file
dialout:x:18:myuser
Then restart. I can now access the devices owned by group dialout.