@John: Congratulations! Good to read the gig went well. You obviously made a good decision buying the GXT

@Jan: The GXT has a gate built in, but unfortunately this can only be set to on or off (default setting = on). You'll have to do something with your mic. What kind of pattern does your mic have? Try using a cardioid type microphone. tilt it up just a bit so that it faces your mouth and is pointed away a bit from your guitar. Condenser mics are great, but TC advices to use dynamic mics with their harmonizers. Try the good old SM58, that one should do well.
For more information regarding your TC-effects, you can also visit the VoiceCouncil Forum

Mike
www.mikewever.nl