How to check if a GPG encrypted file is encrypted using a specific public key

The correct command is gpg --symmetric $MYFILE.

The encrypt option tells it to use public-key encryption. The "id" it is asking for is the id of the public key, which you must have in your keyring. (Usually you use an email address for this, but there are other ways to specify which public key to use.)

Combining the two options, as you did, encrypts the session key with a public key and a symmetric cypher, so that either the private key or the password you enter can be used to decrypt.

Short answer: no

First of all, note that if you are not one of the recipients, it is completely impossible. You do not even have the ability to decrypt the file, much less add a recipient. Even if you encrypted it two seconds ago.

Assuming you are a recipient, it is technically possible. The file is actually encrypted with a session key and the session key is encrypted with your public key, so you could in theory decrypt the session key and reencrypt it to another persons' key, and then package everything together in a file just as if you had originally encrypted the document to both people.

However, gpg does not have this capability. The closest you can get with gpg is

1. Use the --show-session-key option to get the session key (which seems to also decrypt the file, missing the point in this case)
2. Encrypt that session key to someone else's public key (actually, this creates a new session key and uses that session key to encrypt the original session key)
3. Send the both files.
4. The recipient can decrypt the session key and use --override-session-key to decrypt the original message.