I have already synced draft folder, so I see the draft message there. However, when I open the message, I see no option to send it. How can I do that?
WP8's mail client uses its own Drafts and Outbox folders, which are never synched to the server. Anything outside of those folders is opened as though it was received (items in Sent are synched, but still can't be edited). In short, there's really no way to do what you're asking. You could use Forward or Reply All to send the incomplete message, possibly with some addendums at the top, but that's it if you stay within the email client.
With that said, IE on WP8 can load Gmail.com. From there you can go to Gmail's Drafts folder (in the mobile view, it's behind the "Hamburger" menu), finish the message, and send it directly from the browser.
If there's another email client available - one that speaks IMAP/SMTP - then you could use that with Gmail. However, I don't know of any such app (although W10M may provide it). Unfortunately, there's no chance of Google themselves writing it; they have done almost nothing except make the experience worse for WP users by doing things like blocking apps from using their services and shutting down ActiveSync support in Gmail.