That gold goes directly into the guild bank and its per challange, not per person.
It's also limited to 7 dungeon, 1 raid and 3 RBG challenges per week, overall a max level guild is going to earn like 2-3k (total) gold a week from completing all the challeges.
Seeing as you can earn 1k an hour herbing/mining I highly doubt an extra 3k in the guild bank every week is going to affect anything.
