Using GPG without public or private keys
I have a USB thumbdrive and I would like to encrypt/decrypt some of its content. I have put GPG on the drive itself just in case the computer I'm at doesn't have GPG (I have also put on the GPG binaries for windows).
First of all, is there any way to have GPG encrypt/decrypt without using public or private keys and just provide a password?
Secondly, is it even safe to have a private key carried around on a USB thumbdrive with the data it is used to decrypt?