The word used in that context should be gusto.
The used word is wrong.
Is it correct to place used after word? When should I use word used, and when used word?
In the first example, the word used is short for the word that was used, so used is a verb. In the second example, used is an adjective modifying word. In practice, there is very little difference between these, but you must use word used if you're going to qualify it with a reason or location.
Is correct, but:
Is awkward, because you want to qualify used with in that context, so they should be together. The slightly changed meaning is more apparent with commas:
Which is understandable, but again, is rather awkward and would benefit from re-wording.