Unless you have the plane itself in GMAX you cannot add a gauge to it directly.
If not, you could create the gauge in GMAX and compile it to an MDL file using the FSX GMAX SDK. Then use ModelConverterX to Merge the gauge into your VC model (import the VC model, then Merge the gauge). Then using the Hierarchy editor you could move the gauge to the correct location in the VC model (Find it in the Hierarchy Editor, right click on Transformation, Edit Transformation). Finally, export the finished VC.
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