Re: What gauge ?
Ron, Your motor already has a temp buzzer. You might test it to make sure it works. I think the water pressure gauge is more import. If you have the correct pressure, the motor will not overheat, unless the thermostats go bad, which is rare.
I recommend you install a new waterpump and a pressure gauge. Now baseline the gauge reading, so you know what is normal. I get 30+ PSI on my '98 Johnny 150V6, at WOT, and in the 20s at cruise speed.