You don’t ask what would be legal. For it to be safe, the cylinder/bottle has to be in a current of air which will disperse any slight leak. If the carpark bay is walled and roofed in, there might not be adequate air circulation.
Gas is very easy to ignite. Gas in bottles is stored under high pressure and thus all the connections, stopcocks etc. need to be in good condition. Petrol just sits in unpressurised tanks.
If both your petrol tank and your gas bottle started to leak, your car would run dry long before the next barbecue.