Generic_Serial_Device is an interface between DCE C++ framework and Ruby.
On the webpage (wizard->devices->generic serial devices) you will add Ruby code which will be executed when a specific command arrives. Initially it has only 5 or 6 commands (initialize, release, idle, command for child, process incoming data and so on)
To that commands you can add more commands which match your device capabilities (on, off, set level and so on)
So when you send a command from orbiter(or MessageSend or whatever else) the code you put in there will be executed.
So this are short answers on your questions:
1. Yes it does accept ruby syntax because it IS ruby
2. It's ruby man, you can use it the way you like, connect to some server to read mail or spam your neighbor or whatever else your computer and its peripherals can do (the only thing you must be carefull is do not create another classes, because you are already in one and be sure you know what you are doing)
3. You can always get sources from svn and look into them. But we encourage users to use GSD because this is the EASIEST way to create new device which will actually work (don't need to care about memory alocation, thread synchronisation, deadlocks and over minor problems)