Remember there is a different between a document reference object (which GetMessage() is going to return) and a document object.
Once you have an instance of your document reference object, you need to access it’s document property in order to work with the physical document.
Here’s some psuedo code:
set oDocRef = GroupWise.Account.GetMessage("abcdefg") MsgBox oDocRef.Document.DocumentNumber
In your example code it appears you already have the Document Reference object (MyMsg.MessageID), so there’s no need to call GetMessage – it’s going to return the object you already have. All you need to do is reference the Document Reference’s .Document property.
I hope this helps.