I have created a macro file that should produce a Gaussian (Normal) distribution of gamma particles ranging in energy from 0 to 250 keV as follows:
/control/verbose 0
/run/verbose 0
/tracking/verbose 0
/gps/particle gamma
/gps/ene/type Gauss
/gps/ene/min 0 keV
/gps/ene/max 250 keV
/run/beamOn...