How old is that gate valve? If you start taking things apart perhaps take the opportunity to replace it with a ball valve. Assuming there is another valve upstream from it.
We had a 1950's era gate valve fail closed, which was replaced with a ball valve, and another one is currently stuck open, which will be replaced when the main line is. (The open one is not a pressing issue since we also have a shutoff at the curb, and if push comes to shove, the city valve is just beyond that.
At the very least consider rebuilding the gate valve, since the washer in there is probably rock hard.