At the outset, the manufacturer cannot avail abatment - it is the GTA who avails the abatment. The service Receiver, i.e. manufacturer has to go by the declaration given by GTA in Goods Consignment Note and pay service tax accordingly.
In my view, use of Cenvat Credit for discharging service tax obligation on reverse charge basis is now not allowed. This is beacuse Rule 3 (4) (e) of Cenvat Credit Rules 2004 says that Cenvat can be used for payment of service tax on output service. The GTA service for the manufacturer is not "output" service but "input" service and hence, Cenvat on Tele. service, Professional services, etc. cannot be used for paying service tax on GTA services availed.