well thi is the field which am currently working under.
The main advantage of GALS design is that there is reduction is power consumed for executing an instrcution ( in a GALS based micro processor) over all there is reduction of power supply but the main disadvantage is the fact that there is also reduced efficiency -- speed of operation. even though the power reduces by some 12% the speed of operation drops by some 7.5% approx. --- this is for any tupical GALS design.