How to connect grounding wire to gas pipe for 2017 NEC 250.104(B)
There are several types of brass clamps depending on your AHJ. Some will allow acorn style clamps some require the style that has a separate screw for the wire to be secured and the brass clamp on the pipe. Only # 6 is required # 4 is required for concrete encased electrodes but everything else in a residential install # 6 would meet code ( it is ok to use #4 if the clamp is listed for that size).  I am talking about copper wire here.