Liferay: Get Journal Article's DDM Field value

getDDMFieldValue.groovy

import com.liferay.journal.service.JournalArticleLocalServiceUtil
import com.liferay.asset.kernel.service.AssetEntryLocalServiceUtil
import com.liferay.journal.model.JournalArticle
import com.liferay.portal.kernel.util.LocaleUtil


def fieldName = "field name"
def ja = JournalArticleLocalServiceUtil.fetchJournalArticle(id);

def entry = AssetEntryLocalServiceUtil.fetchEntry(JournalArticle.class.getName(), ja.getResourcePrimKey())

def ddmFormFieldValuesMap = entry.getAssetRenderer().getDDMFormValuesReader()
        .getDDMFormValues().getDDMFormFieldValuesMap();

def locale = LocaleUtil.getDefault();

def value = ddmFormFieldValuesMap.get(fieldName).get(0).getValue().getString(locale)

You could get both DDMFormValues and DDMFormFieldValuesMap directly from the Journal Article:
def ddmFormFieldValuesMap = ja.getDDMFormValues().getDDMFormFieldValuesMap();