How to let CA signed x509 key work in OpenPGP mailing system?
The fact that you bought a certificate from an X.509 certification authority doesn't matter to any OpenPGP implementations I know of: when your friend opens the message you just sent them in their OpenPGP-enabled mailer, it won't care a whit about the X.509 CA a priori.

Not only would you have to shoehorn your certificate into the OpenPGP message format, but you would have to teach their mailer software to interpret it, and teach your friend to accept that particular CA.  It might be easier to teach your friend to accept S/MIME, if you really insist on this particular X.509 CA, with the caveat that S/MIME is a dumpster fire of shoddy '90s crypto engineering—or it would be easier to just use OpenPGP, which is also a dumpster fire of shoddy '90s crypto engineering but gets incremental improvements from time to time.