Hi Outrageous,
With the PGP Gateway Email Encryption you do not need to install PGP desktop on the client machine all the encryption and decryption will be handled on server itself. The email flow it will be from Email Client -> Exchange -> PGP Gateway Server.
Please check the below related article:
Symantec Gateway Email - Configuration Requirements for a Secure Deployment
www.symantec.com/docs/TECH223624
Pictured Installation Guide for Symantec Encryption Management Server
www.symantec.com/docs/TECH197003
Managing Mail Routes on Symantec Encryption Management Server (formerly PGP Universal Server)
www.symantec.com/docs/TECH149370
Also the Landing page for Symantec Gateway Email Encryption.
http://www.symantec.com/gateway-email-encryption/