RFC4880 uses ElGamal for the asymmetric encryption and so it's a discrete log problem. Roughly the private key x should satisfy `a=b^x mod n` where b and n are known, and a is part of the public key. It goes through similarly for elliptic curve-based schemes.
FWIW, OpenPGP doesn't only offer ElGamal, and we never use that algorithm. We use Curve25519 by default since quite a while, before which we used RSA. We've never used ElGamal and also don't allow importing ElGamal keys, since they're insecure and deprecated in the crypto refresh (the upcoming update to the OpenPGP standard): https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-cry....
Good point, I just picked the simplest example. In fact I use Proton for my personal email and wouldn't dream of it if I didn't think your crypto was up to scratch.
