In the main word completion window, you will notice two check buttons. One is labeled ``Show Empty Completion Slots'', and the other is labeled ``Insert space after completion''.
The first option, ``Show Empty Completion Slots'' controls whether or not GTKeyboard presents you with empty options. This is merely a visual distinction - if this button is pressed down, then all 10 completion possibilities will be shown all of the time, even if one or more of them is empty. If this button is unchecked, then options that do not contain a valid completion will be hidden from view.
The second option is ``Insert space after completion'' which controls the way GTKeyboard completes words. Normally, if you type q and GTKeyboard suggests the completion quickening, when you choose to insert the completion, the result is ``quickening'', as it should be. If the ``Insert space after completion'' button is checked, instead of inserting ``quickening'' GTKeyboard will insert ``quickening ''. (The same word followed by a space character). For people who use this feature, it is to account for the fact that if a word is completed, the user will usually go on to type the next word rather than adding on to the current word. If that is the case, it can save time to insert a regular space for the user.