Вы можете использовать JSONSlurper из json-lib:
@Grapes([
@Grab(group='net.sf.json-lib', module='json-lib', version='2.3', classifier='jdk15')
])
import net.sf.json.groovy.*
def data = """{
"menu": {
"id": "file",
"value": "File",
"popup": {
"menuitem": [
{"value": "New", "onclick": "CreateNewDoc()"},
{"value": "Open", "onclick": "OpenDoc()"},
{"value": "Close", "onclick": "CloseDoc()"}
]
}
}
}"""
def json = new JsonSlurper().parseText( data )
println json.menu.popup.menuitem*.value