You can add the .zip file for the plugin in your /lib and it will be installed.
Example:
compile ":myPlugin:1.0"
File:
/lib/myPlugin-1.0.zip
Note: You have to zip the content of the plugin folder.
Source: http://grails.1312388.n4.nabble.com/Insert-own-local-plugin-into-build-config-td4646704.html