OP, if you do find a gate valve work it back and forward a few times if it's stiff and don't tighten it down too tightly, a squirt of WD40 on the shaft will help too ... if the HW tap still drips with the gate valve off then don't worry about it just catch the overspill at the tap, gate valves can be like that. Going too tight on the gate valve either open or closed then they can jam and then snap.
Worst case if it does snap closed then if you are confident and skilled enough to take a tap apart and change a washer, I don't think it would be too far a stretch, if needed, to then put a bung in the CW cistern or drain/bale it, disconnect the gate valve's 2 compression nuts and put a lever valve in it's place.
Best scenario, it's a 22mm and the nuts/olives can be re-used. Worst case it's a 3/4" then it's a pipe slice/hacksaw, trim off the nuts, a compression straight with 3/4 olives and PTFE tape and then the new lever valve with the same and you will need a small piece of pipe. Even then it's not a difficult task, just be methodical.
If draining a CW cistern, just remember to open all the HW taps before refilling to avoid it airlocking