Hi Voljanko, SU has a basic ruby installation that comes with it. I think it was 1.8 or something. But it is missing a lot of things. I don't know if anyone has a complete (or partial) list of exactly what has been excluded.
But you can include ruby files from a regular Ruby installation in your plugins. You just need to figure out what files you need, then include them with your plugin and require them in your plugins. Then you can use the methods and classes they provide.
Chris