How do I use GMSKDemod, with a known startup and termination state?

I am implementing GMSK modulation/Demodulation for a GSM system. Each GSM packet starts with 3 known bits, ends with 3 known bits plus guard bits. So, GSM state machine (which considers ISI) starts from and ends in known states. How to implement using Matlab's comm.GMSKDemodulator command? Or, some other convenient toolbox? If I use comm.GMSKDemodulator now, seems it does a 16 symbol traceback, which is unnecessary, and the final bits do not exit due to the 16 symbol delay of the traceback.

Please advise.