With a 20A circuit breaker to one 20-amp GFCI receptacle, can I use two 14 gauge wires, (that are already there), in place of a 12 gauge?

Absolutely NOT!

To use a 20 amp circuit, the cable must be 12 AWG copper.

If any part of the circuit is using 14 AWG cable it is a violation of the NEC code and a severe fire hazard to use a 20 ampbreaker and device.

14 AWG copper wiring can only carry a maximum of 15 amps. 12 AWG AL wire is also listed for 15 amps MAX!

Absolutely NO!

You CANNOT parallel #14 wire to use on a 20A circuit.

What do you mean the #14 is "already there"? Is this an existing circuit?

Thanks again for the replies - Stanfortyman - no its not in an existing circuit. Just happen to have two 14 gauge wires pulled from the panel to that location and won't be used for anything. Was hoping I wouldn't have to open up the wall to pull a 12 gauge. Thanks again!