Hi All,I have a disused gas meter box in

you must call the gas company, whose name is written on the meter plate and ask for its removal - the meter is NOT your property - furthermore it is also possible that the pipe where the meter is connected is full of gas, and by removing the meter, the gas escaping can cause damage and fires

I presume you dont have a gas meter but just the empty box in the wall?  If you no longer have a gas supply,  and the old gas tubes have been removed, then yes  you can take out the box, put in a water meter box and divert your water to the new box/existent hole.   If there is still a gas connection outside the property then you wont be able to do that, and you must have your water connection a certain distance from the gas pipe.    Your plumber will probably not thank you, and depending where you live the water company may have something to say about it too.   It worth asking the plumber, they deal with these things all the time.