Gail is right. You cannot really see the GBR from Coolangatta or the Sunshine Coast. You really have to go north to either Port Douglas or Cairns. Some travel agents may differ, but I have been a number of times, and the only place to go is either Cairns or Port Douglas.
Coolangatta is like the Miami of Australia....crowded beaches and high rises..
It would be much better to base yourself in Cairns, rather than further south..
Let me know if you need any more info
Louise in Sydney