As far as I understood, you have a problem in implementing a variant subsystem. I have attached a simple model of implementing a variant subsysyem.
Once all the subsystems in the variant model are designed then we need to define the corresponding conditions. Which can be done as shown. In the example that i have attached,l have two subsytems and I have named it as "Gain=20", "Gain=10" . My condition is if variable G == 1, then subsystem "Gain=10" will be active and if G==2 subsytem "Gain=20" will be active. In both cases the other substem will be inactive.
The value of varaible G has to be given in the command prompt.