The onscreen keyboard in GTKeyboard functions exactly like a normal keyboard on your desktop. Each of the buttons has its normal function, with the exception of the buttons on the top row, (F1-F12, ESC, Scroll Lock, etc) which do not have any function yet. The shift and caps lock buttons work just like they would on a normal keyboard, as well as all the other symbols like !@#$ etc.
As soon as the application starts, you can begin typing, and the output of your typing will appear in the output buffer, the text space near the bottom of the screen where the guide buttons are at the bottom. Each key that you press will put its corresponding symbol into the buffer at the bottom, possibly modified if the shift key is on. In addition, the NUM LOCK key and the CAPS LOCK key are toggle keys - the status buffer will inform you when they are turned off or on, and they will remain pressed down when on. To release them, just press them again.
On the bottom row of the main window, there are convenience shortcut buttons to perform common tasks such as to select all of the text in the widget, to open a file, to clear all the text out, to quit the application, and so on.
For convenience, there is also a popup menu that you can use with GTKeyboard. Just right-click in the text area at the bottom where you normally type into, and a menu will appear to let you perform common GTKeyboard tasks with, such as hiding/showing different screen elements, customizing fonts/colors, and so on.
Don't bother wasting button presses on repetitive things like quoting certain sections of text! The shortcuts menu in the top menubar will help you out with things like that. If you go into the quoting submenu, and click on ``Quote'', it will produce ``'' and put the cursor between the two quotes so that you can continue typing your quoted text. When you're done with the quoted text, use the cursor keys next to the spacebar to go to the left or right of where you currently are to continue typing.
In the Misc menu, there are also functions to move the cursor automatically to the beginning or end of the file, as well as the beginning or end of a particular line. It will place the cursor appropriately so that you don't have to worry about hitting the cursor keys the right number of times in order to go to the beginning/end of the line/document. They function just like the HOME and END keys on a normal keyboard.
Also, currently with the keypad, you have the buttons Home, End, and Delete. They function just like the keys on the keyboard. The HOME key takes you to the beginning of the current line, delimited by a linebreak, the END key takes you to the very end of the line, and the delete key deletes one key forward from your current position, just like on a normal keyboard.