GPG (along with many other encryption mechanisms) utilizes a method called key-pair encryption. As the name implies, there are two keys. Let us call the keys “A” and “B”. The interesting part of key pair encryption is that any content encrypted by key “A” can only be deciphered with key “B”. The reverse is true, as well. Any content that is encrypted by key “B” can only be deciphered by key “A”.
The general application of key pair encryption is to designate one key “private” and the other key “public”. The private needs to be kept as a top secret, while the public key can be distributed to the whole. The idea is that everyone (including the owner of the keys) can encrypt content using the public key. However, only the owner of the private key can decipher the encrypted content.
In other words, key-pair encryption is suitable not only for encrypting one’s own files, but also for other people encrypting content before transmission over insecure media.