To install GPG run:
sudo apt-get install gnupg
To encrypt a file with a password run:
gpg -c inventorofbitcoin.txt
You will then be promoted to enter a password.
The encrypted file is stored as inventorofbitcoin.txt.gpg note that the original file isn't deleted.
To decrypt a file with a password run:
You will then be prompted for the password and after the correct password has entered you will have to original file.