Already in NEO 2 one can test-run a contract invocation by calling the RPC methods invoke and invokefunction. But this fails if the invocation runs through a CheckWitness() statement in the invoked smart contract. As mentioned in this related pull-request this problem will probably not be resolved in NEO 2 anymore. But it is planned for NEO 3.

Therefore, with NEO 3 the automatic fetching of the GAS consumption of a contract invocation can be implemented. This is a part of the ContractInvocation class and the necessary code is already available but commented out and not covered by tests.