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.