Alas, the blockchain people are the ones who made it popular. I would love to sign into things with GPG keys, but I have never seen a single thing that supports it.
Is it popular, though? Even now, the number of Metamask-enabled applications feels tiny relative to the provenience of GPG signing in the world of software.
No, while it's popular in certain spheres it's decidedly unpopular in general. It is, however, orders of magnitude more popular than GPG for website sign-in.
Right. If we're talking about practical usage though, GPG sees more widespread adoption than blockchain auth. Part of it is because it's older, but also because it's simpler and more redundant.