There are other sources on the net if you search. Local means it runs on your machine. Hamster does the connecting to the Internet. Gravity connects to Hamster like any other server. It can connect to another news server, or multiple servers, and pull in articles.
Mail works the same way.
You can connect to multiple mail servers and distribute the incoming mail or too newsgroups. You can then apply Gravity's rules and scoring to the mail. Another thing I use Hamster for is testing. If you read a lot of isolated newsgroups on different servers you could consolidate them into one list on the local server and read them with Gravity without switching servers. Hamster will fetch the articles.