How to encrypt a mail between two interlocutors?

My friend and I communicate via email, we both use gmail. We want to exchange text and drawings that we don't want to fall into third hands. How do we set up something like GPG? We have never used a desktop client for mail so we need a guide for absolute beginners.

I have an experience of using Jabber with OTG but I don't like chat where messages do not have a topic, I also tried to use TOX but it can lose messages if the recipient is not online - so we need mail encryption with the ability to exchange small files.