Creating Gifford-McMahon (GM) cryocoolers for cooling at temperatures below 4,2 K is a quantum leap in the development of techniques for obtaining helium temperatures. The paper provides a comparative analysis of different types of regenerator packing, which allow obtaining temperatures below 4,2 K. The advantages of cenosphere packing filled with helium are shown. Using a wave approach to modelling regenerator operation, we analyzed operation of a two-stage GM cryocooler, having a cooling capacity of 0,2 Watt at the temperature of 4,2 K. Calculated value of the length of the heat exchanger regenerator with the packing from cenosphere for such cryocooler was equal to 320 mm, i.e. substantially bigger than the length of the regenerator packed with intermetallic compounds of rare earth elements. The resulting length value does not exceed a technically possible size of regenerators for GM cryocoolers. Advantages of cenosphere packing can compensate for the increased size of a cryocooler.