The Gas Emergency Service is on duty 24 hours a day and is prepared to intervene immediately in the event of a gas leak. All reports of Gas Emergency Service intervention are recorded. In the event of any emergency, report it immediately on the toll-free phone number 992.
When gas is leaking:

open windows and doors wide to allow ventilation
close the gas tap in your home
do not use an open flame
do not switch on any electrical appliances or lights
inform people in the vicinity of the hazard and leave the building
do not use the telephone/mobile phone in the flat
call the Gas Emergency Service on 992 from a phone outside