If all applicable limiters are set correctly, then the performance for any steady state operating point can be calculated. Many details about the parameters that affect the engine performance must be known for that.
In the aircraft, however, the pilot does not want (and does not need) to know all the engine control details, he is only interested in thrust and sets his throttle lever to the appropriate position -
Modern engines all have electronic controllers called FADEC (Full Authority Digital Engine Control), EEC (Electronic Engine Control) or DECU (Digital Engine Control Unit), for example. These controllers contain a complex thrust control logic which guarantees that the necessary thrust is delivered, but not more. Delivering more thrust than required would consume unnecessarily engine life and would lead to increased maintenance cost.
In a generic program like <%GASTURB%> one cannot model all fancy control logic features, however, the most typical and most important thrust management methods can be simulated.
There are two basically different thrust management methods: One can either aim for getting the maximum performance from an engine or for getting consistent performance values, independent from engine-