Mailman is very lightweight, has Rails integration, and doesn't require a database to store routes (Astrotrain does). Instead, it uses a Sinatra-like DSL to define routes and conditions.
Gmail accounts can be handled using a MDA like getmail and the built-in maildir support in Mailman.
Mailman works fully with 1.8.6, 1.8.7, 1.9.1, 1.9.2, REE and JRuby.
Are you handling gmail accounts, TLS authentication ? Is this 1.9 compliant ?