Pull Gmail labels via IMAP using Gmail IMAP extensions

run.rb

require 'net/imap'

# Net::IMAP raises a parse error exception when fetching attributes it doesn't
# recognize.  Here I patch an instance of Net::IMAP's `msg_att` method to
# recognize the Gmail IMAP extended attributes

# Refer to:
# * https://developers.google.com/google-apps/gmail/imap_extensions#access_to_gmail_labels_x-gm-labels
# * http://blog.wojt.eu/post/13496746332/retrieving-gmail-thread-ids-with-ruby

GMAIL_USER = 'you@gmail.com'
GMAIL_PW   = 'secret'

# you must enable IMAP on your Gmail account for this to work
gmail_imap = Net::IMAP.new('imap.gmail.com', 993, true)

# patch only this instance of Net::IMAP
class << gmail_imap.instance_variable_get("@parser")

  # copied from the stdlib net/smtp.rb
  def msg_att
    match(T_LPAR)
    attr = {}
    while true
      token = lookahead
      case token.symbol
      when T_RPAR
        shift_token
        break
      when T_SPACE
        shift_token
        token = lookahead
      end
      case token.value
      when /\A(?:ENVELOPE)\z/ni
        name, val = envelope_data
      when /\A(?:FLAGS)\z/ni
        name, val = flags_data
      when /\A(?:INTERNALDATE)\z/ni
        name, val = internaldate_data
      when /\A(?:RFC822(?:\.HEADER|\.TEXT)?)\z/ni
        name, val = rfc822_text
      when /\A(?:RFC822\.SIZE)\z/ni
        name, val = rfc822_size
      when /\A(?:BODY(?:STRUCTURE)?)\z/ni
        name, val = body_data
      when /\A(?:UID)\z/ni
        name, val = uid_data

      # adding in Gmail extended attributes
      when /\A(?:X-GM-LABELS)\z/ni
        name, val = flags_data
      when /\A(?:X-GM-MSGID)\z/ni
        name, vale = uid_data
      when /\A(?:X-GM-THRID)\z/ni
        name, val = uid_data
      else
        parse_error("unknown attribute `%s'", token.value)
      end
      attr[name] = val
    end
    return attr
  end

end

puts "logging in to Gmail..."

gmail_imap.login(GMAIL_USER, GMAIL_PW)
gmail_imap.select("INBOX")

puts "downloading all msgs in INBOX..."

gmail_msgs = gmail_imap.uid_search(['ALL']).collect do |uid|

  # pull some attributes, including 'X-GM-LABELS'
  gmail_imap.uid_fetch(uid, ['RFC822', 'FLAGS', 'INTERNALDATE', 'X-GM-LABELS']).first

end

puts "\nGmail Labels:"
puts '-------------'

gmail_msgs.each do |msg|
  puts msg.attr['X-GM-LABELS'].inspect
end

puts "\ndone."

Line 54 should be "val =" instead of "vale =".

Thanks for the monkey patch.

Would this help?
http://www.offlineimap.org/doc/use_cases.html#sync-from-gmail-to-a-local-maildir-with-labels