Importing GPG Key

zcat /path/to/file.sql.gz | mysql -u 'root' -p your_database

> will write the output of the mysql command on stdout into the file myfile.sql.gz which is most probably not what you want. Additionally, this command will prompt you for the password of the MySQL user "root".

The usual means of verification is to contact the key owner and ask him/her to provide you (over the phone, or in person) with their key fingerprint. If you have strong reason to believe you are actually speaking to the person identified by the key, AND if the fingerprint the person provides matches the fingerprint of the key you have, THEN you can be quite certain you have the owner's real public key (and NOT a counterfeit key, created by an imposter for the purpose of impersonating that person).

Once you have verified the authenticity of the key to your satisfaction, you should sign that public key with your own private key. Once you've signed the key, you will no longer get those WARNINGs, because by signing it, you have indicated that you believe the key to be authentic.