First start the server and then have the client connect to it and let the good times roll.
Assuming you are in the grpc-java root folder you would first start the route guide server by running
$ ./gradlew :grpc-pollService:pollServer
and in a different terminal window then run the route guide client by typing
$ ./gradlew :grpc-pollService:pollClient
That's it!
Please refer to [Getting Started Guide for Java] (https://github.com/grpc/grpc-common/blob/master/java/javatutorial.md) for more information.