Service tax liability in your hands will be 100% under reverse charge mechanism. Abatement allowed for GTA service is 75%. Payments made by you to goods transport agency will attract service tax @ 12.36% on 25% value charged by GTA. Effective rate will come to 3.09% but please take care while filing returns as these days returns are getting rejected because of using such shortcut rates. Please remember .. there is only one rate of service tax i.e 12.36%.
For getting more clarity on GTA, kindly read my Article, which also got published by CENTAX Publication in R.K. Jain's Service Tax Review STR dated 15-01-2014.