Ondrej,
No, go is not constant:
L = inductor value
Fsw = switching frequency
D = duty cycle
Sn = on-time inductor current slope times the current gain Ri (1/gmps) = 
Se = slope compenation of 296,000 at Fsw = 698kHz for TPS54040A. This value is directly proportional to Fsw and depends on circuit internals.
Ri = 1/gmps = 1/1.9 = 0.526 ohm
For Vin = 15V, Vout = 8.8, Iout = 50mA, Fsw = 698kHz, I calculated go ~= 0.0533.
With gmps = 1.9A/V and Rout = 8.8V / 50mA = 176ohm, GpsDC = 32.2V/V or 30.2dB. This is quite different from the value I would get by simply multiplying gmps*Rout = 50.5dB.
These equations are taken from the Vorperian PWM switch current model for peak current mode control.
Hope this helps,
Kris