Gas meter has been moved and I want to know if I can extend existing 10mm earth or have to run a new one.

Because of the use of the term "Earth" I'm going to guess that you are not here in the U.S.A.

Normally, we do not connect the gas meter casing or piping directly to earth. In the U.S.A. all piping is "Bonded" together to keep it at the same potential. The bond is in turn connected to the grounding conductor that connects the earth to the main service.

If the meter and associated piping is connected directly to earth, then the possibility exists that the gas equipment may be at a different potential that all the other metal associated with the electrical service.

No. By US National Electrical Code, the grounding electrode conductor (Earth) must be one continuous length with out a splice or joint.

"must be one continuous length with out a splice or joint"....
See NEC 2011 - 250.64(C) for the conditions allowing a splice.
Rules and exceptions may be different outside of the US. Based on the use
of "10mm", this is not in the US...