Priest of Gix

• WWizards of the Coast2011-01-25
  Official Ruling
  You get the mana whether you want it or not. If you don't spend it, it will disappear at the end of the current step (or phase).
• WWizards of the Coast2011-01-25
  Official Ruling
  The triggered ability isn't a mana ability. It uses the stack and can be responded to.

Rulings via Scryfall