Do local building codes/regulations control where to locate a gas meter and responsibility for moving it?
Building departments usually rely on a nationally recognized model code. In California at the time your house was probably built, this was likely the Uniform Building Code published by the International Council of Building Officials. I don't have any old UBC references, but the current International Residential Code for One- and Two-Family Dwellings which has in part superceded it has an entry related to under floor access that is very much like what appeared in older UBC codes.


  R408.4 Access. Access shall be provided to all under-floor spaces. Access openings through the floor shall be a minimum of 18 inches by
  24 inches (457 mm by 610 mm). Openings through a perimeter wall shall
  be not less than 16 inches by 24 inches (407 mm by 610 mm). When any
  portion of the through-wall access is below grade, an areaway not less
  than 16 inches by 24 inches (407 mm by 610 mm) shall be provided. The
  bottom of the areaway shall be below the threshold of the access
  opening. Through wall access openings shall not be located under a
  door to the residence. See Section M1305.1.4 for access requirements
  where mechanical equipment is located under floors.


It seems a reasonable person would say placing an immovable object in front of this access would violate the intent of the code, but this sounds like it will devolve into a legalistic pissing match where legal assistance could be required to get your way. This could end up costing more than simply paying to move the meter.