A Read Receipt is quite the misnomer: the best you can say is that the message was shown on a screen. Which screen though? GMail is accessible through the webmail, POP3, IMAP, mobile devices (GMail native client, Exchange Sync), mobile web... what exactly are you trying to prove happened?
I've found Read Receipts to be near useless, while Delivery Receipts are generally poorly implemented and probably achieve the same thing: some systems will say that they've taken receipt of the message into the system, but what you actually want to know is that it has successfully arrived in the user's mailbox. Signing for a courier delivery is exactly that, but you'll never know if the recipient ever opened the package.
If you're so keen to know whether the person opened the mail, ask them to confirm receipt manually. Having a computer tell you that you read something is at best trivial and at worst misleading. For a one-line e-mail,this facility may be useful and mostly true, you're likely to have absorbed it. A 1000-word essay, what you really want to know is not just that it was displayed on some screen (and reading that much on a mobile is pointless), but rather that the end-user absorbed everything. Only the user can attest to that.
On principle, keep read receipts out of GMail (or other providers, even Exchange!), and these problems go away. Just ask for a reply that your message is received, understood and/or acted upon. Much more useful.