Can a gas pipe/tube run under floor joists?
Piping run concealed in finished spaces has to be installed through the joists and protected from physical damage. Or be run in black or galvanized steel. Or you could box the pipe run and shield it. Also, a flexible appliance connector has a length limit of 6 feet. 

Code quotes are from the 2012 IRC

SECTION G2415 (404) PIPING SYSTEM INSTALLATION


  62415.7 (404.7) Protection against physical damage. In 
  concealed locations, where piping other than black or galvanized steel is installed through holes or notches in wood studs, joists, rafters or similar members less than 1.5 inches (38 mm) from the nearest edge of the member, the pipe shall be protected by shield plates. Shield plates shall be a minimum of 1/16-inch-thick (1.6 mm) steel, shall cover the area of the pipe where the member is notched or bored and shall extend a minimum of 4 inches (102 mm) above sole plates, below top plates 
  and to each side of a stud, joist or rafter.


SECTION G2422 (411) APPLIANCE CONNECTIONS


  62422.1.1 (411.1.1) Protection from damage. Connectors and tubing shall be installed so as to be protected against physical damage.
  
  62422.1.2.1 (411.1.3.1) Maximum length. Connectors shall not exceed 6 feet (1829rnm) in overall length. Measurement shall be made along the 
  centerline of the connector. Only one connector shall be used for each appliance.
  
  Exception: Rigid metallic piping used to connect an appliance to the piping system shall be permitted to have a total length greater than 6 feet (1829 mm), provided that the connecting pipe is sized as part of the 
  piping system in accordance with Section G2413 and the location of the equipment shutoff valve complies with Section G2420.5.


Have you considered a drop ceiling in this area?

Good luck with your project!