The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From Matthew Schwade:
Whether you use Gmail for your personal or business e-mail, iReply for Gmail gives you the ability to set an Auto-Responder message for your e-mail account. iReply for Gmail is the only app that is specifically designed to connect to your Gmail account and allow you to set the Vacation Responder to on or off, right from your iPhone. This one of a kind app allows you to enter a custom reply message, a reply subject, and determine whether you want the auto-replies sent to your contacts or everyone.Using the easy-to-use iPhone interface, you'll be able to keep those that e-mail you updated on your current status at all times.It doesn't matter if you are on vacation, out of the office, or in a meeting iReply automatically so they all know!