It depends.
If you are using GnuPG's command line it is really hard..
But why?.
There are lots of easy to use mail client integrations:.
Enigmail for Thunderbird (https://www.enigmail.net/)
GPGTools for MAC(https://gpgtools.org/)
gpg4o for Outlook (https://www.giepa.de/produkte/gpg4o)

Just try it - it is not as hard as it is repeated on the web all the time.

Hi, I am the development lead of gpg4o - a GnuPG integration for Outlook
I can tell you that there is no dead easy way for getting GnuPG into Outlook. You are facing three major problems
- Outlook hooks are very tricky especially if your manipulating mails
- There are many dialects of OpenPGP message formats
- Hiding the complexity of GnuPG for the avarege end-user, so that a non-nerd can use it.


We spend several man-years with research and development for polishing our product.

These are the reasons the Open Source Plugins for Outlook are failing in the daily use.

Tobias