You may need to supply some more information. "Flux" could have several different meanings, depending on the context.
- "flux meter" usually means a device for measuring magnetic fields.
- "flux gauge" often means a heat flux gauge for measuring heat flow.
- "flux" can be a material to aid in soldering, so I suppose there migth be a gauge for measuing this material.
- the car in "Back To The Future" has a "flux capacitor", with a flux meter attached.

There may be other instruments called "flux gauges" that I'm not aware of. Since both "flux meters" and "flux gauges" come in many brands and models, you most likely will have to go back to the manufacturer to obtain a specific manual.
Tim F