Check online at great lakes skipper they have all sorts of old new stock, that should be a Faria gauge. RPM gauges never seem to work right. You could try and diagnose, clean connections and also if you can swap the plugs you can see if the fault follows which will show it's an input issue and not a gauge. Giving them a few swats of the hand often helps as well.
I was able to find my multi gauge for fuel and engine data but it took a lot of searching. Mine wasn't from GLS but start there at least.
You can also try the magnet trick. Use a strong magnet around the bezel to exercise the needles. I find that mine are a bit sticky when I open the boat but after some runtime or the magnet, they loosen up and are pretty accurate.
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I was able to find my multi gauge for fuel and engine data but it took a lot of searching. Mine wasn't from GLS but start there at least.