I'm no expert, but how's this:  It looks to me like you're trying to use the element type <STRING> to convert a numeric value returned by FS (an ADF radial) to a text value which the <TEXT> element will display on your digital gauge. 

     In looking through GAUGES.XDR I see that the content of sub-element type "String" included under the element type "Text" is defined as text only.  There's only one other definition in the XDR for an element type named "String", and that one is under the attribute named "PointsTo".  Its content is an enumeration type and must be either "North", "East", "South" or "West".

     And then lastly, I can't find any references to the delimiters/operators you've used in your <STRING> element in any of my XML references.

Bon chance,