Simple cycle and open cycle are the same thing. When gas turbine is run in stand alone mode i.e, its exhuast energy is not utilized it is called open / simple cycle.
Any generating set will have a maximum continious rating. At this load the unit operates at maximum efficieny. There is certainly a scope of generating more power but it will be at the cost of sacrificing the inbuilt margins which is called as peak load operation.
Neelesh