What does the parameter "Gauge Pressure" mean?

Do I have to pay attention by the input of pressures,

because I have to distinguish between the input of static and

difference pressures or is there always mean the static pressure.

Absolute Pressure = Gauge Pressure + Operating Pressure

In fluent, the operating pressure is defined in: Define->Operating Conditions

All other pressure inputs are then Gauge Pressures (like in Pressure Inlet BC).

You have to be careful. The solution is calculated in Absolute Pressure (think of Ideal Gas Law... P=Rho*R*T... if P is in Gauge Pressure, then when P is close to the operating condition, Rho approaches infinity... therefore P must be in Absolute Pressure during the solution). If you forget to set your operating pressure and then input 3atmospheres as your inlet pressure, then Fluent is going to think the pressure is actually 4atm (because the default Operating Pressure is 1atm) and that can have a big impact on your solution.

Hope my explaination is clear.

Good luck, Jason