Without going too off-topic...
Carl is right - there is no way to transmit Centre, Left and Right (and Sub) channels over a single SpeakerLink connection.

If you really wanted to run just one Cat6 cable from the G68 to the 5200HC and you didn't need the RS232 portion of SpeakerLink, you could use the RS232 twisted pair to transmit the second pair of audio channels. A 'mutant' SpeakerLink if you will. HOWEVER, this would require bespoke adapters at either end of the Cat6 cable (as it's no longer true Speakerlink) and would not be very future proof, e.g. if you upgrade to a SL centre you would need a completely different adapter at the speaker end.