Block Gmail Due to Spam

I have no prejudice of Gmail users, but these days my A/I mailbox frequently receives spam emails from Gmail addresses, commonly with no content or just one or two words or emojis. Therefore, I have to set mail filter rules for Gmail mails. Since today, if you use Gmail (username@gmail.com) to send emails to me, you must satisfy one of the two requirements below:

1. You encrypt your email using OpenPGP, and ensure that your subject is also encrypted;
2. You use the string I_AM_NOT_SPAMMER as your subject;

otherwise your email will enter my spam box.