For PGP, whose encryption is fundamentally similar to what SSH does with public keys, public key servers are offered. Through them, the goal is to propagate public keys as widely as possible.

It's infeasible to reverse-engineer a private key from a public key. In fact, that's the whole point of public-key cryptography: given appropriate key lengths, it just can't be done, and the data will be secure no matter how widely the public key is shared.

(Note that "it just can't be done" is dependent upon certainly widely-held assumptions about higher mathematics not being proven false. But if that happens, you'll have bigger things to worry about...)