Practically you have paid 1700 to GTA, you should charge RCM on 1700.
500 was unloading and hamali charges which you can show as labour charges and RCM is not applicable on unregistered inward supplies till March 2018.
However logically since you have paid to GTA entire amount, you should charge RCM on entire amount. Legally only 1700 is traceable so RCM should be paid on 1700.
It is more of a practically problem with practical solution that should work here.