What's needed is that Alice, who knows nothing of crypto, gets mail from Bob. It's encrypted, but there is a reassuring icon of a padlock or something along with a simple plaintext message from Bob, a la 'Hi Alice - Bob here. I've decided to start encoding my email to keep it secure from prying eyes on the internet. I use a reliable free service called (something cool). You can install it in about a minute by clicking on the icon, and after that everything is automatic. If you're not sure, just give me a call and I'll confirm it's legit. your friend, Bob'.

People are totally willing to install stuff and recommend it to their friends as long as it's simple and transparent in operation. Most encryption systems haven't been, because most crypto nerds are skeptical of anything that doesn't expose its functionality...whereas most ordinary people don't want to know how it works, they just want to know that it does and that it's safe. Frankly that's why I don't bother using crypto right now, my academic interest in it peaked when PGP was new and I was passing the code around and joking it was classified as a munition. but since nobody has broken into my mailbox in the last 20 years, I can't be arsed to stick public key blocks at the end of every message and have people assume I'm paranoid or, to do tech support for my relatives.

If you like crypto and think more people ought to use it (which I agree with), maybe a gmail extension would find an audience.

Also email has been around a long time, but the main way of getting someones email is still break into someone account by phishing for their passwords. If the attacker logs in to gmail using your own password, encryption is not going to help (assuming you're envisioning a gmail that does automatic encryption of email and must decrypt it when you view your emails in your browser).