is the gui backend coded width direct windows system calls or there is a layer ? I'm thinking about the work to be done for other systems (linux, os x, ...)
As rebolek said, yes, there is a layer. The blog states that
`Flexible backend support that can be mapped to virtually any kind of UI library.`
which means that the abstractions used are good enough to support different OS.
There are plans to support View (gui) on each platform (desktop/mobile) that Red supports. But it's going to take some time. Lots of other features to be added as well :D .
There is internal layer (View) that calls Windows directly. Work has already started on support of other GUI backends (OS X and Android currently).