Does a gas fire leave smoke?

When it burns gas a gas fire produces hot invisible poisonous fumes that include the odorless gas Carbon Monoxide. Those fumes must be vented to the outside of the building using a good chimney or a properly designed flue pipe otherwise they can easily kill you.

If a gas fire is properly set up it should not produce any visible smoke at all so if your gas fire is producing smoke you must urgently call a licensed gas fitter to fix the gas firefor you.

If visible sooty smoke comes off a gas fire that could be as dangerous to your health as the normal odorless and invisible fumes produced by a properly set up gas fire which vents them through a proper flue pipe or chimney.