Can I fill the dry gauge with a fluid?
The only difference between non-filled and the filled gauge is the glycerin fluid itself. I've read that glycerin can be bought at most pharmacies and is found in the aisle with laxatives. If you fill your own gauge, be sure to leave 10% air inside.

How do I reset the vacuum gauge, so the needle reads zero when there is no pressure on it?
Vacuum gauges often develop internal pressure during shipment when there are large changes in atmospheric pressure. The internal pressure must be released for the gauge to remain accurate. If you have a gauge without liquid filling, you can cut off the tip of the rubber stopper, and it should reset the needle to zero. Be sure to cut enough off the top of the gauge so that a tiny hole is visible in the cap. If you have a liquid filled gauge, be sure to do this while the gauge is in the upright position to prevent the loss of fluid and do not install the gauge any way other than vertical.