You'll certainly need a gas fitter to remove the gas heater. (It's a serious criminal offence for anyone other than a Corgi-registered fitter to disconnect a gas appliance).
An electric heater can simply be plugged in to a convenient socket but, if there's not one available, you might want to consider getting an electrician to permanently wire it in. If you've not got a ring main running near to the location of the heater you'll have to decide whether you want the cable to run, say, along the surface of your skirting board or to be hidden behind the plaster. The first option would be far cheaper (but not as aesthetically pleasing) because it would avoid having to cut into the plaster, with the subsequent time and expense of replastering and repainting that part of the wall.
Chris