Hello everybody,
As you know, google does not officialy support PGP in their web mail service. There are multiple turn arounds to accomplish this, here are two of them :
The first way is to download a free (as in FLOSS) PGP replacement called
GnuPG and use an extension called
FireGPG (on Firefox).
A second way to do it is to use a free (not FLOSS) service called
Freenigma, which is a web based solution (but you still have to use a firefox extension. The obvious downside of using freenigma is that of trusting them with your private key!