This BASH script downloads all gravatars which are available for the mail adresses stored in the macOS contacts application.

gravatars-from-mac-os.sh

#!/bin/bash
OLD_IFS=$IFS

IFS=$'\n'

databases=( $(find ~/Library/Application\ Support/AddressBook -name "*.abcddb") )
for database in ${databases[*]} 
do
  echo "Database: $database"
  mailAddressesString=$(sqlite3 $database "SELECT ZADDRESS FROM ZABCDEMAILADDRESS;")
  mailAddresses=( $mailAddressesString )

  for mail in ${mailAddresses[*]} 
  do

    mailMD5=$(md5 -qs $mail)
    image=$(curl -s "http://www.gravatar.com/avatar/${mailMD5}?d=404&s=2048" | base64)

    OUTPUT_COLOR=''

    if [ "${#image}" -gt 100 ]
    then
      mailForFileName=$( echo $mail | tr . _  )
      echo $image | base64 --decode > "${mailForFileName}.jpg"
      OUTPUT_COLOR='\033[0;32m'

    else
      OUTPUT_COLOR='\033[0;31m'

    fi

    NO_COLOR='\033[0m'
    printf "${OUTPUT_COLOR}${mail}${NO_COLOR}\n"
  done
done

IFS=$OLD_IFS