Restricted sender list
The Restricted Sender list allows you to prevent specific email addresses, or any address from a specific domain, to send messages to groups. They won't know they've been restricted, but you can choose to receive notifications.
Login, click to select the group where you want to make changes, click Settings:
Scroll down to Message Flow and click Manage restricted senders:
Click Add to either add individual email addresses (1), or allow anyone from a specific domain (2) by adding *@domain (as a wildcard email address):
Click Add restricted senders and Gaggle Mail shows you the added email addresses/wildcards:
