There is an attack against RSA if you encrypt very small values with a small exponent and insecure padding. GPG generates a large symmetric encryption key, and uses that to encrypt the message, uses 65537 (as is considered good practice) as its exponent, and uses a cryptographically appropriate PKCS#1v1.5 padding scheme

As far as I'm aware, there's no reduction in security by encrypting to multiple public keys (other than increased risk of compromising one of the keys).