Funny - I've toyed with the idea of fitting one (even though I don't really need one either!).
You could fit a valve tap on the end to cut off the gas, or a stop-end to blank it off. I am assuming that its an 8mm copper solid pipe here. If not you can get a spigot that converts a flexible pipe to a threaded gas fitting and then add your tap / stop-end etc.
I'm no expert but it can be done.
(You may be lucky and have a gas set up with individual gas taps fitted for each appliance to be able to isolate them independently. If you have then you can simply turn off the feed to that appliance! (I'd be tempted to wired it closed or tape it off with a warning note though to prevent accidentally turning it back on until a more permanent alteration had been made though).
As to whether the gas blow air heater can fit in the same space.... that's by no means certain, as you have to worry where your exhaust pipes and air ducts will route.
I moved my diesel blown air heater from under the sofa to the back of my van, and could not fit it exactly where I wanted as I'd have needed my air inlet and hot exhaust to run through the diesel main fuel tank! Check what is around and indeed under your space before starting to cut holes!
