Note, this information is taken from user Big Ed at StackOverflow. Thanks Ed!
Create a properties file myExternalProperties.groovy and put it on your classpath (such as the $TOMCAT_HOME/lib directory).
Create a configuration file grails-app/conf/MyConfig.groovy to use the external configuration values (if needed). You will not be able to use the properties defined in myExternalProperties.groovy within grails-app/conf/Config.groovy.