Set is a factor 0-100 of what your wastegate spring is. If you tell me what car you have we could go somewhere with this.
Gain depends on your type of wastegate control. Are you actuator or divorced? It controls how fast boost comes in. If it's set too high boost will surge and spike uncontrollably. Actuators use low numbers where divorced wastegates tend to use higher numbers. Mine is a divorced wastegate set at 28. My friends STi is set a 8 (actuator type)
I forget boost start right now....
Warning is obvious, if you hit the warning level (usually in kg/cm) then it beeps at you and flashes.
limiter in some cases will reduce the boost by the specified value when you hit your warning. In others it is the value that you want for your boost limit and will reduce boost a set margin after hitting it.