MySQL Connector/J's own Project Object Model (POM) file specifies a transitive dependency to Protocol Buffers (protobuf-java) since it is required for using X DevAPI.

MySQL provides connectivity for client applications developed in the Java programming language with MySQL Connector/J, a driver that implements the Java Database Connectivity (JDBC) API and also MySQL ...

Download MySQL Connector/J 5.1.6 from here. Extract the bundle and copy “mysql-connector-java-5.1.6-bin.jar” to the “lib” directory of Grails application.