I figured it out. Here's a little VB.NET code snippet in case anyone else needs to know how to manipulate values not directly exposed by the gData library. This returns the XML node (and creates first it if it doesn't exist). I actually change the value through the innerText property.
Private Function GetFileAsObject() As XmlNode
For Each ext As Object In _contactEntry.ContactEntry.ExtensionElements
If (ext.GetType() Is GetType(XmlExtension)) Then
If ext.XmlName = "fileAs" Then
Return ext.Node
End If
End If
Next
Dim doc As New XmlDocument
doc.LoadXml("<gContact:fileAs xmlns:gContact='http://schemas.google.com/contact/2008'></gContact:fileAs>")
Dim node As XmlNode = doc.DocumentElement
Dim newExt As XmlExtension = New XmlExtension(node)
_contactEntry.ContactEntry.ExtensionElements.Add(newExt)
Return node
End Function
This link helped immensely: http://code.google.com/p/google-gdata/wiki/UnderstandingTheUnknown