Gate valve won't open (water heater)
Replacing that valve is not that big a deal. I will assume you have copper tubing, usually 3/4" for a water heater.
The cheapest way is to sweat solder in a new ball valve. But if your soldering skills are suspect, you can do it with compression fittings.
ball valve- threaded
Compression to male pipe coupling
short nipple - brass
water heater flex connector
Turn off water at meter, cut out old valve. Remove the female adapter from the inlet of the water heater. Install the male pipe end of the compression fitting into the valve. install the nipple into the other end of the valve. Ball valves are marked with a direction, observe the correct flow. Connect the compression fitting & valve to the supply pipe. With the valve closed turn the water back on and verify no leaks. Use the flex connector to connect from the valve to the water heater.
Do the hot at the same time, no valve required, and it will be duck soup when you get ready to replace the old water heater.
Coastal South Carolina