I have a 180g....it took my GT over 6 months to get to 5"....and it all the sudden freaked out and beat the tar out a my Short body ZZ flowerhorn of the same size.....GT is now at a pet store up for sale I will tell you that the GT got to 3" from 1" very fast, but when it hit 4", growth slowed down a lot. Diet was great, frozen bloodworms, Krill and Hikari Bio-gold+.... so not sure why he stopped growing as quick as he was.
I bought mine at 1 inch 2 years ago, now it is only 7-8 inch, and he is in a 190.
I change a lot of water and feed a lot, the fish is also not under stress. Don't know what it is with GT's, but I seem to have very slow grow rates with them.
Just for comparising: it took my Midas less then 4 months to grow from 1 to 7 inch..