Both FFX and FFX-2 have Cactuar Gatekeeper sidequests. Are you sure you're all talking about the same one?

In FFX you intiate the sidequest you examine the stone south of the Cactuar Valley, as redxiii said. Clues are given to the locations of the cactuars, all of whom are on Bikanel Island, or there abouts. You need to walk (do not use the Airship) to these locations, find the Cactuar in question, then return to the stone you started at. Once all you have obtained all 10 spheres from the Gatekeepers you need to examine the stone to cause a scene where the sandstorm ceases and you can go into the valley to grab your rewards.