The GDM format is the file type used for messages in the inbox from The Sims Online's Play Test version and onwards. The Pre-Alpha, however uses the XML format, to which there is a template in every version which is also an XML file.
Messages can be edited offline, with programs such as notepad++ and a Hex Editor.
The format for messages is different in the GDM format, however, it's same format, but requires a hex editor to edit., unlike the XML. There is a value that determines what type of message it is in both formats. In the XML it contains: <Message id="41" type="3" read="0">. The GDM contains hex code, which goes like this: 04, which is the type.
There are multiple types of messages.
08 makes the text invisible, which a grey box as the face, and you can add them to the permit/ban list, but doing so causes the game to crash. This isn't the case when clicking "Where Am I?", 00 has the same effect. 07 seems to be an admin, but with icon, it seems to be unknown at this rate. 06 seems to be the same as 05, but it's icon are 4 house instead of 1. 05 seems to be the same as 06, but with 1 house instead of 4, maybe this could stand for "roommate"? 04 stands for The Sims Online Staff. 03 stands for Maxis Team. 02 stands for EA Games Team. 01 stands for the same as 02, but these two messages may be color coded as "warning" and "final warning".