List current Principals in group Managed Service Accounts
It turns out that you can list all the properties for gMSA by running:
Get-ADServiceAccount -Identity <gMSA-account> -Properties *
And if you want to narrow down the list you can use:
Get-ADServiceAccount -Identity <gMSA-account> -Properties PrincipalsAllowedToRetrieveManagedPassword
It's not very readable, since it's a list of distinguished names and has several other properties listed, but it's a useful command.
Update: to show all the entries from this properties you can use this command, which is shorter and easier to handle that what @Gregory posted
(Get-ADServiceAccount -Identity <gMSA-account> -Properties *).PrincipalsAllowedToRetrieveManagedPassword
You can select specific property, instead of the wildcard *, to decrease the data flowing over the network, but the line becomes prohibitively long due to the verbose name of the property.