OpenPGP specifies PKCS#1 v1.5 for RSA padding. Attacks on PKCS#1 v1.5 have been well understood for over 20 years[1]; every few years, someone finds a new one.
RSA itself is well-known for having weird number-theoretic problems that implementations have failed to respect, to catastrophic effects. Best practice for algorithm selection is to pick algorithms where users can't compromise the integrity of the scheme through poor public parameter selection; RSA forces the user to pick a public modulus and exponent, leading to all kinds of silly things that actually happen[2].
Edit: Correcting myself: most attacks on v1.5 padding concern encryption, not signatures. The general fragility argument remains, however.
