Use iccgamut to create a .gam file and a 3D HTML document of an RGB working space profile:

iccgamut -v -w -ff -ir rgb_profile.icc

View the gamut:

Repeat again using a destination CMYK profile:

iccgamut -v -w -ff -ir cmyk_profile.icc

View the gamut:

Then use viewgam to compare the two gamut files with the larger RGB gamut shown as a wireframe:

viewgam cmyk_profile.gam -w rgb_profile.gam output_comparison.gam

View the gamuts: