Nick Pike was going to do a whole series of tutorials over at FS2X.com, but after the first three there's been nothing, I don't know if he's still working on them. Check them out for more detailed info.
Here's a quick run down, I'm just a beginner too.
What happens when you click on it (obviously)
Every part of the XML gauge is an element. Defines that it should do something, more data is to come, etc.
When the gauge will not work (obviously)
><Gauge ...........> All the XML Code for a gauge starts
>with it and ends with </Gauge>
> Used to enclose all the XML code and
>show the name and version.
> Example "<Gauge Name="Weight" Version="1.0">"
Used to give more detail as to how an element should move or rotate, used with nonlinearity or rotate.
Allows you to define what happens when the mouse moves over, clicks, drags etc
Used to allow elements to move
Used to allow elements to rotate
Allows for text gauge using strings, digital readouts, etc
Used to tell which parameter is being called, also sets minimum and maximum values.
When the gauge is visible (obviously)
>Thanks for your effort.
>Roelof Kruijer (Beginner)