There are a few ways to sex GBRs, as previously mentioned:
1. pink stomach on female
2. black spot missing "jewels" on a male (although I have a male right now that has those jewels at the edge of his black spots, but does not have a pink stomach)
3. the black on the pectoral fins (sometimes quite pronounced on females)
4. the dorsal rays. Males typically have a couple of their dorsal rays, the 2nd/3rd IIRC, that are much longer than the others. And by much longer, I mean that: one of my males has his so long that they are laying over top of a few other rays.
5. Shape of the head. Males will typically have rounder/snubbed noses while females have elongated heads.
With that being said, just using one of the above to sex a GBR is not a good idea, although the pink stomachs have been the easiest for me --> the sub-dominant females still have some pink speckling on their stomachs IME.
Have fun with them! They are fun to watch. You may even get a pair to form while in the QT and they might hoard/take control of the clay pot, so keep an eye out