Since 3 December 2002 when Interior Ministry's decree of 22/11/02 came into force and was published in the Official Bulletin 283 of 03/12/02, LPG vehicles equipped with safety systems compliant with UN/ECE regulation 67-01 may park on the first level of a car park, even if there are other underground levels.
LPG vehicles equipped with safety systems compliant with UN/ECE regulation 67-01 are almost all those which have had the system installed since 1 January 2001, when compliance with the regulation became obligatory. To check whether your vehicle is compliant, look at your car documents, which should contain the words "EQUIPPED WITH UN/ECE 67-01 SAFETY SYSTEM".