scrypt is asymptotically much more expensive to crack.
And, what's the reason to prefer both over PBKDF2?
scrypt is asymptotically much more expensive to crack.
bcrypt is asymptotically marginally more expensive to crack than PBKDF2, but not enough to matter; I'm guessing tptacek's point here is that bcrypt has more library support available (despite PBKDF2 being the de jure standard). I wouldn't say there's a strong argument in either direction.
Could you make a recommendation for or against using GPG, since that's by far the most common approach for asymmetric signatures?
Avoid if possible. The code was written by a colony of drunk monkeys, in an era before anyone understood the basics of modern cryptography; I'm really not sure which is worse between gnupg and OpenSSL. Of course, GPG is the standard for encrypted email, just like SSL/TLS is the standard for web sites, so you may have no choice...