I think the easiest approach is to put the zip file in the lib project folder and then add the entry to BuildConfig.groovy. For instance:
1. 'grails-image-tools-1.0.5.zip' placed in lib.
2. runtime ":grails-image-tools:1.0.5" added to BuildConfig.groovy
Since the dependency manager is also looking inside the lib project folder, I don't need to worry about setting any paths, etc.
EDIT:
Grails, , 2.1.1. , @Saurabh Grails 2.4.3