I had another experience in dealing with GZ agent.
I had a booking of flights A-B-C-D with A-B and C-D being connecting flights and all in "i" class. I wanted to change flighs A-B to say flights E-F. The agent told me there was no "i" class left in flight C, and that she could not amend my booking. She advised me to cancel my original booking, hoping to release 1 "i" class of flight C to the system so that she could simultaneously help me rebook flights E-F-C-D in "i" class.
I agreed and cancelled my booking online. She then discovered a "P" class was released to the system for flight C, so that ticket price went up. I told her to proceed with booking flights E-F-C-D in fare class I-I-P-I, but she couldn't do it due to flights E-F violate some maximum layover rule. She also couldn't revive my original booking (A-B-C-D) as Class "i" was no longer available for flight C. I was a little bit annoyed at this time because she did advised that E-F-C-D is bookable (albeit may not be all Class I). After expressing my discontent, she spent few hours asking supervisors etc. and finally retrieved my original booking with the same itinerary and price.