Have you checked the results of "grails dependency-report command" ?

You have to know which lib is adding this transitive library and then excluding it with the exclude closure.

For example, for a Grails 2.5.5 project, I want to use the itext 2.1.7 and the plugin rendering is using an older version. So, I did this:

dependencies {
       runtime 'com.lowagie:itext:2.1.7'
}

plugins {
        compile (":rendering:1.0.0") {
            excludes 'itext'
        }
}