GnuPG is an open source implementation of the OpenPGP standard. PGP is not in widespread use. GnuPG is not a good tool to add PGP support to other software.
That's a perfectly valid complaint. Your casual dismissal seems very wrong to me.
Depends on what industry/domain you work in. The reason PGP isn't more widely used is: It's expensive, it's complex to the typical "user" and businesses don't want to invest in real PKI even if they need it. (People are lazy and cheap...)
Add in the factor that most developers don't have the chops to figure out and implement nicer software based on the mess that is GPG. You have a few serious tools wrapping it used in ETL applications and ilk like Gpg4Win and the Bouncy Castle libraries.