Southern California Gas Co. customers will get a little extra present this Christmas--if the state Public Utilities Commission approves. The company said it wants to pass along, as credit on its December gas bills, $25.6 million in refunds it received from gas suppliers. Actual refunds will vary depending on the individual customer's gas consumption. But the company estimated that an average user, consuming 690 therms of gas during the 12 months ending in October, would receive about $2.19. The utility serves the area from Visalia to the Mexican border, with the exception of San Diego County and Long Beach.