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Explanation: It seems like they've inserted the names of groups from their company address book. Using Microsoft Exchange Server (mail server; where the address book is probably housed) as an example, it is a recommended best practice, when creating groups and other directory objects, to follow a standard naming convention. Exchange can use Active Directory groups as email distribution groups. Groups are just one of several types of objects, and the recommended convention is to place a G before the name of each group. G can also refer to Global Group, which is one type of group in Active Directory.
So it could be as simple as the company network naming convention, and this is something you should ask the client.