Matlab GMSK vs GSM... How close?

GSM first takes difference bits, then does GMSK modulation on that. I cannot tell what Matlab's http://www.mathworks.com/help/comm/ref/comm.gmskmodulator-class.html and http://www.mathworks.com/help/comm/ref/comm.gmskdemodulator-class.html do. The GMSK demod includes a trellis decoder... so maybe it is decoding the difference bits back into original bits? Anyone know more about this comparison? Have code implementing GSM encoding/mod/demod/decoding in Matlab?

Thanks, Tom