I have a gamma 2 installed, its a very nice unit for the price amount, Only thing I would like it to have is a dual siren mode but to answer your question.
The gamma 2 works like this
in the standby mode you can hit the man button and you can cycle it thru 3 tones( you can set each tone to what you want)
on the tone mode you can set it to 1 tone that you like and if you hit the man button you can also set that to a tone you want
so in all you can have 5 tones of your choice..
I have mine set to standby+man1 wail: man2 yelp: man3 pier: tone button powercall: tone+man button mech Q
you can also set the horn mode to 3 different sounds of air horns
system is very easy to use.. plus I like the added 2 lighted swicthes
Down fail to this model: is only 100 watts output( I would use a whelen siren, I had another speaker and it just wasnt that loud)
it doesnt have a dual siren mode & it is not back lit so at night sometimes its a pain.
But I would buy another one if I needed a good cheap siren!!!