I've had numerous gt's and have bread them and I find that it is actually fairly easy to sex a good specimen when it is young. Males will often have better colour in the dorsal fin...not the red area, but the area below. They'll have more of the blue green spangles there whereas females have relatively little colour there, just pale green. Your fish doesn't show much colour there, so I think it's female. Also, females tend to get those little black spots on the operculum when in a mood, and they're pretty prominent on your fish. Males don't often exhibit those spots.