Best way to set beam properties in geant4

I am doing simulation for particle traveling in free space (after the beamline). I calculated the RMS beam size and angular divergence from the twiss parameters right at the beamline’s exit. I want to simulate what happen to the beam after the beamline.

I consulted the manual and figured out the possibilities with /gun and /gps .However, the manual does not help as it just give table without further explaining the options (maybe i am too new to the field so i didnot get them).
So what is the best way to set beam properties from these calculated quantities ?