You might want to take a look at this page: [[Dryer Smells Like Gas|new_window=true]] It addresses the whole range of these problems.
There are a few model specific issues as well which are less common. Older dryers with the two solenoid side by side gas valve (Whirlpool/Kenmore and others) can occasionally have problems with the seals on the plungers inside the valve developing cracks which allow gas to seep through. These can leak even when the dryer is off. Replacing the coils won't fix this. A new gas valve is the best bet
Some LG models from about 10+years ago had an occasional problem with the control board failing and allowing the gas to turn on without the igniter turning on as well.