Recently tried to create a new group. The desired name was taken. Out of curiosity I went to check out the group with the desired name and found that the account was 4+ years old, no commits, no projects, no nothing. Just created and left.
Is there a protocol for flagging such things? Is Gitlab ever willing to take such action? Admittedly low priority, just curious.
Welcome to the forum! 
If it satisfies the requirements to be considered as a dormant namespace, you can claim that namespace by contacting the support team.
As far as I know, there is no protocol for flagging, but if your intention is to use that namespace, you can use the above protocol.
Thanks! This is the exact information I was looking for (except that the FAQ does not provide an explicit way to request the dormant namespace). 
EDIT: did some cursory investigation and found a way to submit support ticket: https://support.gitlab.com/