GateCycle doesn't do transients. Modelica does transients, but is impractical for multiple steady-state cases. On a single project I typically run 150 to 250 cases with GateCycle. It takes way too much time and effort to set up a model in Modelica and get it to converge to a useful steady-state result for more than a few cases. GateCycle is shockingly expensive and Modelica is free. Which model to use depends on what you need it to do. Thermoflex / Thermoflow / GT master is also shockingly expensive, but has some advantages in that it will assume all sorts of stuff for you if you don't know much about the system you're trying to model. Ebsilon is obscenely expensive, but has some additional capabilities. PEPSE (also very expensive) has a lot of capability, but building a single model is a HUGE effort, requiring a LOT of time. In a nuclear plant, they're used to everything costing a fortune and taking forever, so that's why PEPSE is often the choice for modeling the steam system in nuclear power plants. All of these models will require a huge effort on your part to learn how to make them do what you need them to do. The first question to answer is: Do you need transient analysis or not? The second question is: How much money can you spend on this?

for modelica: check the modelica.org page for teaching material!