Sending from !GNUsocial to !Twitter requires that your !GS instance is running the #Twitterbridge plugin. Check with @neadmin. The Twitterbridge is not well supported, since Twitter keeps changing its API. Inbound Twitterbridge messages tend to create resource issues on GS (several well-connected Twitter users can flood the GS database with Twitter messages), and Twitter doesn't play well with software that tries to show its messages outside of its own environment. Twitter is like the roach motel of microblogging: Messages can go in, but they can't come out.