Gmailのgemで件名をエンコーディングする

これで上手くいった。

# coding:utf-8                                                                           

require 'gmail'
require 'base64'

gmail = Gmail.connect("YOUR_GMAIL_ACCOUNT","YOUR_PASSWORD")
mails = gmail.mailbox('YOUR_GMAIL_LABEL').all
latest = mails.first
subject = latest.subject
subject = subject.gsub("=?ISO-2022-JP?B?","").gsub(/\?\=$/,"")
subject = Base64.decode64(subject).force_encoding('ISO-2022-JP').encode('UTF-8')

puts subject