Licenced plumber broke my gas line while digging for my drain line

Ask the plumber to provide you with the name and contact information for his or her insurance company. Such a request is often enough to get them to pay up. A licensed plumber is probably qualified to make the repairs themself, but if you have any qualms about the competency insist on having someone else do the work. And be sure to have the work inspected - gas lines are dangerous is not repaired properly.

It should be the plummer who broke the gas line.

The plumber should repair the line that he broke. You may want to call the gas co. if you have problems. You could also contact the CSLB and file a complaint.