Gas Generator how many watts!!!

Basically, add up all the wattage of EVERYTHING that will be running, and if anything (like a fridge) has a motor, I would double it's rating because it needs more to start.

Start with about 2KW (kilowatts) (a kilowatt is a thousand watts), so, 2000watts "just a rough start"
You can maybe get by with less if the above is all you are going to run.

MAKE SURE that you TEST everything before you throw it in the truck, for generator exhaust noise AND possible electrical noise induced into mike and guitar pickup cables

MAKE SURE you take a length of wire and some sort of ground stake and ground the generator frame.

YOU MAY KNOW that most all traditional mikes and "phone jack" hi-Z cables are GROUNDED to the amplifier, and if there is some ground fault in the power system, you can find yourself with a "hot" mike stand or guitar.