MySQL Connector/J is free for usage under the terms of the specified licensing and it runs on any Operating System that is able to run a Java Virtual Machine.

MySQL Connector/J 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) ...

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.