FS Panel Studio has tools to help you create a new XML Gauge by starting with an existing XML Gauge. Select the Gauge: Create a New Gauge menu pick. This will start a Wizard which will step you through the necessary actions.
Making a new Gauge from scratch is not a simple task. It's easier to start with an existing Gauge, copy it to a new Gauge file, and then edit it to look and act like your desired new Gauge.
FS Panel Studio's Wizard will step you through the process of selecting an existing XML Gauge, copying it to a new Gauge file, and then starting the Gauge Editor. For more details, see the Tutorial.
NOTE: Starting with FS2004, Gauges can now be located in the Aircraft's individual Panel folder. If you wish to use these in other aircraft you must first copy the Gauges to the common Flight Simulator Gauges folder.