Even Gmail would be a safe service to use if people just copied/pasted locally encrypted blocks of data into/out of email messages.
I suspect that even if everyone could be convinced that encrypting everything was a good idea, the moment gmail couldn't collect and profit from the contents of people's private messages they'd shut the service down. It exists only to exploit us.
Gmail would have to decrypt the message to show it to the recipient. Once it's in plain text they can do whatever with it.
If the user has to copy/paste a blob from gmail into a file and then run gpg on it, well Microsoft Windows is scanning everything on your hard drive and maybe even in RAM so they'll get the decrypted file.
A handwritten letter is probably the most private way to communicate with another person who you can't talk to directly.
