current.groovy

pom.withXml {
    def newDep = asNode().dependencies[0].appendNode 'dependency'
    def g = newDep.appendNode 'groupId'
    g.setValue(project.group)
    def a = newDep.appendNode 'artifactId'
    a.setValue('version_plugin')
    def v = newDep.appendNode 'version'
    v.setValue(project.version)
    def s = newDep.appendNode 'scope'
    s.setValue('runtime')
}

desired.groovy

pom.withXml {
  asNode().dependencies.appendNode {
    dependency {
        groupId { project.group }
        artifactId { 'version_plugin' }
        version { project.version }
        scope { 'runtime' }
    }
  }
}