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What do you mean with 1Gbps UTP line? A pair of unidirectional differential signals? Usual GBit ethernet 1000BASE-T is e.g. using 4 STPs, operated bidirectionally through hybrids. 1000BASE-CX (short-haul copper) is a standard using 2 STPs undidirectionally for up to 25 m distance. Both standards are achieving a data payload < 800 MBPS, because they are using 8b/10b coding.
To multiplex 100BASE-T connection with user data, the ethernet data have to be decoded first.