The amp meter should be right at zero or very slightly on the + side. The alternator is putting out just what is being used, no more or no less, so the gauge stay centered. Mine will show plus charge for a short while right after cranking it after setting for a week or so, after that it's centered.
Gas gauge needs to have 6 volts going into it, and the fuel sender needs a really good ground wire right to the sender. The tank is not a good ground. I ran my ground wire all the way back to my main ground stud I installed when making my wire harness. Fact each light is grounded that way also, I don't like chasing ground issues! Use a jumper wire from sender to frame ground or to the battery post as a test, if it still doesn't work, check input voltage at the gauge.
