Yes we normally treat a GSM tslots (TCH) the same as a fixed wire 64KBs ccts, etc etc.
There is some question as to whether Erlang B or C should be used, but generally in the Industry, Erlang B is used.
I have worked in GSM for 4 years with a number of companies and all have dimensioned TCH this way.
That said, there are some special situations, where Erlang B is used, but in a different way, specifically fractionally loaded networks. Here TCH’s on the BCCH carrier may be considered separately to hopping TCH’s in another group of channels.
Hope this helps