GPUs don't follow Moore's law.
Yes they do. Transistor density in GPU (and thus hashing power) has doubled roughly every 24 months. If anything GPU development more perfectly follow Moore's law.
Do you mean potential hashing power? Because transistor count and hashing power are not directly correlated, as noted by the 2billion transistors of RV870 monstrously crushing fermis 3billion transistors in hashing power.
+1 Moore's law doesn't really apply to GPUs because for their different architecture nature than CPUs, Moore is more into RISC and CISC architectures.