I am going to assume that GPG is already installed on your computer.
With that in mind, let's take a look the commands you need to encrypt and decrypt any file using GPG. To encrypt a file the command structure is;
gpg [OPTIONS] recipient filename
For simplicity I am going to show you how to encrypt a file for your own personal use. This means that only you, or anyone who has your personal gpg keyphrase working on your machine, can un-encrypt the file.
In this case the recipient name will be your personal username you use on your Linux machine. So the actual comand to encrypt a file will be:
gpg --encrypt --recipient 'USERNAME' filename
This will create a new file called filename .gpg
To decrypt the file use the command:
gpg --output filename --decrypt filename.gpg
When you encrypt a file you will not be asked for your passphrase.
When you decrypt a file you will be asked for the passphrase used when you create your key.