Hi TB19
I have some experience with GBRs. They do not do well in a newly cycled tank, as they prefer a more mature tank, you are right they are very sensitive to water conditions. They need pristine water conditions that include vacuuming and water changes about every three days. They do best in soft water with PH of 6.0-6.4. Most information say that the water temperature should be between 72-77*. My GBR that I have had for about four months is in water that is at 81-82* and doing fine.
The most important thing is that they prefer the more acidic, soft water and a lower PH level than what you have in your tank.

~ kate