GUIs are somewhat difficult. The first thing you need to do is upload pictures to the sprite manager that you want to use. Then, set up the pictures as buttons in your GUI. Then, use the script to explain what you what to do.
Here's an example of a GUI in my game. It is GUI #9 on my list.

The coding is as follows:
if (interface == 9) {
// They clicked a button on the death screen
if (button == 0) {
RestoreGameDialog();
}
if (button == 1) {
RestartGame();
}
if (button == 2) {
QuitGame(0);
}
}
Now, if button 0, the "Restore" button is clicked... the restore game dialog (The load function) shows up. If the middle button occurs, the game restarts. If the Quit button is pushed, the game quits without asking if you want to quit (Hence the 0 after "QuitGame").
I'll explain a bit more later. The help file can be quite useful for finding more advanced commands that I can't figure out.