Lara,
ARS (Automatic Routing Selection), is the routing table, tells the switching system or PBX where to route the call depending on the numbers dialled. Say, if routing says block all calls with 00%, It will block all the international calls.
Where as GoS (Grade of Service) is the service level offered by an operator. Say, an operator has P.02 GoS, means two calls fail out of 100 simultaneous call attempts.
regds.