why the differential phase/bit is pi/2 in GMSK baseband Moduration

GMSK->(f2-f1)T = 1/2   --> (2*Fdeviration)T=1/2 --> Fdeviration*T=1/4 --> 2*pi*Fdeviration*T=1/4*2*pi --> Wdeviration/baud=pi/2

//fourier