With a typical Gossamer Mail setup (ie. using a catchall), you won't be able to use SpamAssassin at the mail server level as it will use the catchall account's SpamAssassin config.

You can however, move the SpamAssassin checking into Gossamer Mail. With the SpamAssassin plugin, this allows you to do this, but at its current state, the plugin only allows a minimal SpamAssassin configuration. It can be easily extended to allow the user to change all aspects of their SpamAssassin config (it isn't like this by default as most of these settings will overwhelm most users).

Adrian