Read with Notification no 11/2017-Central Tax (Rate) Dated 28.06.2017 which again amended by further notification no 20/2017-Central Tax (Rate) Dated 22.08.2017. GTA having an option to charge 5% GST with no benefit to claim ITC or 12% with ITC except providing services exempt from tax by notification no.12/2017-Central Tax (Rate) dated 28.06.2017.
To make it simple.
12% GST by GTA---------- ITC can avail by GTA
5% GST by GTA--------- NO ITC benefit to GTA.
As per above rule, you should charge GST through out the year on your sales @ 12% so as to avail ITC benefit on purchase of Truck..