No. GPG doesn't use the public key to do the actual encryption, it generates an intermediate key, uses that to encrypt the message and encrypts that intermediate key with your public key.
Maybe this is too far-fetched and it's beyond what is technically possible today, but wouldn't the decrypted message be the plaintext for the session key and wouldn't then the session key basically be the plaintext for the public key? I suppose it's considered unbreakable because you only get one plaintext per session key?