UV rays....
Jeff, this is usually caused from extensive exposure from UV rays. There is things on the market that you can use that will help take the fade out but for the most part, it's in between the inner and outer layer of the gage.
These gages are also hermetically sealed to keep out any humidity. If you have seen gages that get moisture in them, then fog up where you can't see them, then it's because that seal has been lost.
Your gage is hermetically sealed with an inert gas, argon. It is possible for you to remove this glass, break the seal and with precision, cut another piece of clear plastic or safety glass to re-install. You'd have to put two tiny holes in it, like 6 and 12 o'clock to use a small amount of argon to flood it before you plug the holes.
For the most part, it's kinda fruitless, since the time it takes in repairing it is not worth the time and money it takes to just replace it.
You can look on ebay and find these things fairly regularly. You can also go to ebay and look for a store called "great lakes skipper" and ask him. He has a "jet ski junkyard" and buys and sells parts from non-running skis........
Good luck..........:cheers: