different questions about waveguide simulation in HFSS

Hello
I designed & simulated a wr90 Waveguide in HFSS and I'm going to analyze output results so first of all I want to know what exactly "Gamma" is in HFSS? because I plot it's real and magnitude but it seems it is different from

.

and we know that S11=Gama when port 2 is matched but why magnitude of S11 is not as same as Gama

I will appreciate for your help

'Gamma' in HFSS is the propagation constant.

Also, your S11 doesn't look spectacular - is your simulation just a single length of wr90?

Thank you PlanarMetamaterials
Yes I excited only the first mode (TE10) and the length of wr90 is fixed also note that I plot the magnitude of S11 not dB(S11).
and dB(S11) is

or without negative sign?it means the best dB(s11) is the minimum ones? or not?

Thank you.

S11 measured in dB will not have a minus sign. The definition you gave is for return loss.