OG - I want to learn how to do this. I went to the PGO Signature link above, and in that thread they use Kleopatra. Must we use Kleopatra to put a PGP Signature on a message? Is that the only vendor?
No, you don't have to use Kleopatra. This is just one program as a recommendation in the tutorial above. But why wouldn't you use Kleopatra desktop application? If you are not experienced, it is easier to go through all the steps mentioned in the tutorial than to search for alternate solutions.
If you still want to encrypt a message in a different way, I'm sure you will find a bunch of similar tutorials online.