You have a lot of options for encrypting messages in Gmail. If you want to keep your encryption within Google's ecosystem, tech blog Digital Inspiration shows off how to use Google Docs to encrypt your outgoing messages.
Digital Inspiration's trick uses a custom Google spreadsheet to encrypt the message. Then you'll need to share a password with the recipient so they can decrypt it on their end. You certainly have easier options out there, but this one doesn't require external apps or extensions. Head over to Digital Inspiration for the full guide.
How to Encrypt your Gmail Messages with Google Docs [Digital Inspiration]