my 6" male GT i had, had no nuchal hump. lack of or having a nuchal hump is not an indicator of sex. the only certain way is to vent (flip the fish upside down and look at its genital region) oo = male and oO = female.
common indicators ive read that are used by people seem to be (as far as im aware none are entirely accurate):
the amount of blue lines on the face (less=female if i remember correctly),
width of the orange/silver band on the tail (wider = male for instance),
pointed fins/trailers = male,
head shape,
plumpness which could indicate eggs or just a well fed fish.
im not sure of the sex with yours though, your GT seems to be a mixed bag.