The GPS can select from multiple, weighted sources event by event.
The following will generate 10 events, approximately half of which will be muons and the other half pions.
# muon /gps/source/intensity 1. /gps/particle mu- /gps/position 0 0 0 /gps/direction 0 1 0 /gps/energy 10 GeV # pion /gps/source/add 1. /gps/particle pi- /gps/position 0 0 0 /gps/direction 0 1 0 /gps/energy 10 GeV /run/beamOn 10