my civic trembles and smells of gas everytime i turn on the heater, what can it be?

You sure its the smell of gas and not the smell of the heater coolant? If its gas you might have a leak into your heater fan bay. If its coolant obviously your coolant pipeline has a weak spot that probably leaks under pressure from the pump. As for the trembling, either the pump system is cranky or your engine is getting cranky and even turning on the heating system is putting too much strain on it. Those are my guesses anyway.

Your exhaust manifold is leaking, you better get that fixed because you can die from carbon monoxide and NOx poisoning from the emissions

for the trembling check your motor mounts

look for a leaking fuel injector and /or fuel rail leak i had a dodge that the injectors only leaked at start up when the ambient temp was below 20 outside as the car warmed up they stopped leaking!!