GPSS might look a bit odd to you if your computing experience began relatively recently -- say, after Windows 95.
The Whole Screen
GPSS takes over the entire screen. This is because you want as much screen real estate as possible for maps.
You won't see a window with title, minimize, maximize and close buttons. You won't see a topmost toolbar with dropdown menus for File, Edit, Tools, Help, and so on.
Just type a letter...
Instead of using graphical menus, you interact with GPSS most easily with mnemonic keystrokes (for example, you tap 'd' for 'destination,' 'w' for 'where are we?' and so on), rather than gesturing, pointing, clicking or otherwise using general Windows user interface conventions.
This kind of key-based interface harks back to early forms of user-friendly computing, and you'll soon get used to it.
(In fact, many graphical programs are torture to bad mousers, and most good programs include keyboard shortcuts for that reason. With GPSS you get the keyboard shortcuts without the often irritating point-and-click approach.)
You might say that all free things come with a price. The price with GPSS, other than the interface, is that most users have to set up (or at least tweak) their own maps, guidance directions, landmarks, and towns and cities.