GMSK Demodulation

Hi All:
    while looking at GSM literature, i found it quite interesting in how
GMSK is demodulated.
    In simulink library, the baseband GMSK demodulator included a hard
decision viterbi decoder. In my mind viterbi decoder is used for error
correction of convolutional encoded channel data. In one GSM simulink model
i was looking at, after demodulator(which included viterbi decoder), the
data are passed to differential decoder-> de-interleaving -> and to viterbi
decoder again! 
   So what's the purpose of using viterbi decoder in GMSK
demodulator?(this is what simulink does). Also , some literature talks
about MLSE equalizer, is that replacing the viterbi decoder ? for error
correction?

Thanks
sun