Class G is an amplifier circuit that allows for higher temporary power/watts on music peaks. Two B+ rail voltages are used. One is low voltage and one is higher. On music peaks the higher voltage rail is switched on. Afterwards the low voltage rail resumes operation. My Hitachi receiver is Class G. It can double its power on peaks, from 50wpc to 100wpc. Class G is an economic way to have a higher power amp without the cost and weight of a robust (more capacitance) power supply.
Mcintosh class g amp compared to Mcintosh class ab amp
Why are mcintosh home theater amps are class g instead of class ab. What is class g, is it better than ab.