Need way to extend subwoofer cable  

ya
go to radio shack and get an RCA to wire connector (i dont know what its called, but it connects regular copper wire into an RCA cable.

are you using an old reciever and a new sub? is that why you need to use gauge cable instead of RCA to RCA?

You shouldn't be feeding an RCA input with a speaker wire. You should be feeding the sub from either an RCA output (and shielded cable w/RCA's on it) or a speaker-level output to a speaker-level input using (what else?) speaker wire.

You could always buy some 75 ohm coaxial cable and put 2 RCA end connectors on it. This is essentially a subwoofer cable.