Exhaust smoke

White smoke commonly related to a coolant in the exhaust due a head gasket failure or other failure
monitor coolant and check if its getting low

white to gray smoke is an indication of oil burning that may be due to a bad valve seal or other mechanical engine failure

know you say that have a smell of gas
check if have oil mixing with oil

If the oil contains fuel-check the injectors and fuel pressure regulator if its leaking