You can easily tell the differences between a male and a female GT. It's moderately difficult until they are of age, or about 3" or larger.
The male has long, flowing fins that will sometimes exceed the length of his caudal (tail) fin. His color will bright, and his body will be slender but stocky (in typical cichlid fashion). I think that only the male has blue coloration on his gill plates (do not use this particular indicator when selecting young as the LFS).
The female will have rounded fins that are not nearly as long, flowing, and pointed as the male. Her body will be plump, and will not grow to be as large as the male. Coloration will be bland, lacking the sparkly color the male has.
Hope this helps