Note that Hamster has changed since I wrote this. Newer versions use another script but some of it may be similar.
Gravity can read email if it is translated to news format and posted to a news group.
You can do that with Hamster, a third-party small local mail and news server. This page describes how to set up Hamster for this purpose. I don't want to give a new Gravity user the impression that they need to install Hamster. Most users will have no interest or need. If you are not familiar with basic network terminology and configuration files you may have a tough time.