Hey Katana, unfortunately it is impossible to estimate how much they will stretch. Sometimes you can have 2 plants of the same strain and one will stretch a lot more than the other one. To be on the safe side, you want to expect them to double their height during the flowering stretch. And don't be afraid to defoliated some of those fan leaves that are blocking light to your lower buds. That will help get you more quality tops on them. Good luck!
Generally plants will do a large stretch for about a 2 week period. After that some strains mainly high in sativa varieties will continue to grow but at a very reduced rate. In my experiences with GG it doesnt continue much after the 2 week period and I've had very similar experiences with Zkittlez.... so I feel it's safe to assume u got about a week more of stretch to go and that will be ur end game size minus bud girth of course lol. Stay lit fam.