Smell of gas because of incomplete combustion. Cold start engine idles a lot faster, typically above 1K rpm, to get up to operation temperature quickly. You probably smell gas when back up and this is because you "drive into" you own exhaust. The smell goes away when engine is running at normal temperature.
Car "jerks" as result of higher than normal engine idle. I'd check my fluid levels: engine oil, tranny fluid and coolant just to make sure.