Did you get your hands on a private key somewhere? Is it asking you for a password if you try to use the key in some way?
Let's talk about how to crack that password so you can use it.
First lets create a key to crack:
The generation process imports the key automatically, lets view it and make sure it worked by running:
Cool, now that we know it generated, lets export it to a file. This is similar to what happens when you come across a priv/pub keypair on a fileshare or something:
Now we have the private key (which actually includes the public inside it as well) in a file. At this point, an attacker would download this file locally and run John The Ripper on it.
The first thing the attacker needs to do is convert it to a john friendly format. The jumbo pack version of jtr has a tool called gpg2john:
Then crack like normal with JTR: