There is a excellent panel program from Flight one that allows you to edit or create panels, and will also allow to to edit the XML gauges as well you can either donload it or buy the cd for less than $40.00.
This program really works great and you can even trouble shoot panel cfg files and it will let you know of any erros. It comes with a well documented help file in PDF format. Make sure you have that if not you can dwonload it from ADOBE.