An error with gzip

Have an issue with the following snippet of code, in particular the execution of the `gzip -9 ${ARCHIVE_FILE}`.
It is failing with a ReturnCode of 1 - Can anyone lead me to a souce that identifies & describes what RC's there are for gzip, as I've not been able to find any.

  echo '-- TARing up '${RUNTYPE}' files ...'

  tar -cVf  ${ARCHIVE_FILE} \
            ${File1}        \
            ${File2}
  TAR_RC=$?
  if [ ${TAR_RC} -ne 0 ] ; then

    echo ${0}' : ** ERROR w/- '${MKT}'('${RUNTYPE}') ARCHIVE file.'
    echo 'Error Code: '${TAR_RC}
    ##- Alert Notification -##
    EMAIL_WHO=${EMAIL_SUPPORT},${SMS_ALERT}
    EMAIL_SUB="${0} :  ** ERROR w/- ${MKT}(${RUNTYPE}) ARCHIVE file."
    EMAIL_BDY="An error has occurred with the creation of the ${MKT}(${RUNTYPE}) Archive file (${ARCHIVE_FILE}).\n\nPlease investigate and resolve accordingly.\n\nError Code: ${TAR_RC}"
    Alert_Email

  else

    ##- Continue with GZIP after successful TAR -##
    echo ''
    echo '-- GZIPing Archive(.tar) file ...'
    echo '-- TAR File: '${ARCHIVE_FILE}
    if [ -f ${ARCHIVE_FILE}.gz.old ] ; then
      mv ${ARCHIVE_FILE}.gz.old ${ARCHIVE_FILE}.gz.older
    fi
    if [ -f ${ARCHIVE_FILE}.gz ] ; then
      mv ${ARCHIVE_FILE}.gz ${ARCHIVE_FILE}.gz.old
    fi

    gzip -9 ${ARCHIVE_FILE}
    GZIP_RC=$?
    if [ ${GZIP_RC} -ne 0 ] ; then
      ##- Problem w/- GZIP -##
      echo ${0}' : ** ERROR w/- '${MKT}'('${RUNTYPE}') GZIP file.'
      echo 'Error Code: '${GZIP_RC}
      ##- Alert Notification -##
      EMAIL_WHO=${EMAIL_SUPPORT},${SMS_ALERT}
      EMAIL_SUB="${0} : ** ERROR w/- ${MKT}(${RUNTYPE}) GZIP file."
      EMAIL_BDY="An error has occurred with the creation of the ${MKT}(${RUNTYPE}) GZIP file (${ARCHIVE_FILE}.gz).\n\nPlease investigate and resolve accordingly.\n\nError Code: ${GZIP_RC}"
      Alert_Email
    fi

The `gzip` step executes as expected when performed manually.

Any assistance appreciated.

if a script works fine when run manually and breaks when run from cron, this usually suggests an environment issue --- most likely PATH ... i did a quick check of "man gzip" and didn't see anything there that would make the gzip command in the script break in non-interactive mode