I thinking about GPG, is very widely used, but is secure? Why and where we using GPG?
I found an article from 2002, but is very old.
OpenPGP was initially meant for emails. It works well for that and plugins are available for various emailing software. The sore point of secure emails (encryption and/or signatures) is about making sure that you know the correct public key for any correspondent. OpenPGP relies on the Web of Trust concept, which is neat but, in practice, does not work well (it requires a thoroughly redundant graph of cross-user certifications, that is unlikely to ever exist). If you can make sure that you have the right key (e.g. by exchanging the public key fingerprints in person or over a phone call), then OpenPGP works well (I do that with customers).
Other usages include validation of software packages. For instance, this is integrated in the format for Debian packages and thus used in some major Linux versions (including Ubuntu).