To do this, first create your user-defined gap-fill by clicking on the button on the textivate home page:
Then close the user-defined gap-fill edit box. This adds [#] in front of the words (or groups of words) that you have selected as gaps.
Then type your clue or prompt after the # on the textivate edit screen (in the "Text" box or the "Show all" box), like this:
[#your clue here]the_gapped_word
You might end up with a text that looks something like this: