An utter rip-off of a product-do not buy.
The inference on the product was that it was a 'stand-alone product ready to be plugged into your campaign" so you can add a given element (in this case, Mexican spell-casters) to your system as desired. This is untrue.
What you get a is thumbnail sketch of a western town ( a hundred words or so) , the information that some bandits (with names like Buffalo Annie) are hold up in a icy cave nearby, and a short paragraph describing the Brujaria. And then this gem of an instruction on the last page:
The Gunslingers and Gamblers rules do not make allowances for magic and supernatural
powers. Instead, the GM should use magic strictly as a plot device. The odds are that the
player characters themselves will know almost nothing about magic and whatever they do
know is likely to be wrong. A GM should keep this sense of mystery in mind whenever he or
she introduces a supernatural element to the game.
Yep, that's the 'plug in to your campaign' for adding Mexican witches.
Do not waste your money.