I am replacing some of the galvanized plumbing in our basement and shut of the main supply line pictured below. After cutting away some of the cold water line I have a steady stream of water (filled 5+ gallons so far) and I’ve opened up the water faucets on our second floor. I assumed it was coming from the gate valve, so I tried to tighten it a bit since it’s seemed a bit corroded. It turned a bit and started to turn a little easier after the initial torque—but now it just continues turning. I can hear the water sounds to shift from a varied subtle flow as I turn the valve. At one point it stops but then starts to trickle through again. Does this mean the valve is bad and needs replaced?
If so, will the water continue flowing after I cut away the pvc to remove the valve and attach a new one? 
Plumbing – Well-pump gate valve replacement
plumbingrepair
Related Solutions
First, a silly question for you. Are you sure the valve is not a 1/4 turn ball valve, and are you sure it has not turned off the water supply to the tank. If the answer is no to both questions, then we have to assume the valve is defective and we go to plan two. Now you have two jobs ahead. First, replace defective valve with a 1/4 turn ball valve and second, replace the wax ring with rubber flange. Before you can attack the valve replacement, you must turn off the main water supply, usually found at meter or pressure tank. After you have turned off the water supply. open a couple of faucets that are below and above the valve you want to replace. This will drain water from the system and avoid a lot of water held in pipes above the toilet valve from rushing out all over the place when you unsolder or cut the old valve out. Without seeing exactly what configuration your valve is attached by, I cannot advise you if it can be unsoldered. Another option may be to secure the body of the valve with a wrench and see if you can remove the cartridge retaining nut with a separate wrench. This will avoid the fear of ruining the copper in the wall. you may be able to replace just the guts instead of the entire valve. A pic of your set up would be helpful.
You need to find the main shut off valve in the road, side walk or somewhere on or near your property. They are usually placed in mini manholes with easy access:
But if you're unlucky you will need to get some building plans and try to find where on the plans the supply pipe runs and find the valve at the connection point to the main supply. In most cases it will be near the connection point of the main supply:
but if its not, or if you struggle to close it call your council to help you. You don't want to break that!
NB
It is easy to snap these valves- if its rusted badly apply (known as WD-40, LN-40, HT-40) spray oil based rust release (spray leave an hour try again.. repeat a few times), TAP the valve with a hammer (!TAP!, do not whack it, to help release the valve. Also, usually opening the valve slightly helps to release it then try to close it.
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Best Answer
YES- the symptoms you describe indicate a stripped or broken stem or gate:
YES- in fact, you cannot repair or replace the valve unless you shut off the flow upstream. If you try, A LOT of water will flow.