Once you have made it to Kalebo III, you will have to participate in the hoverboard race to earn an item for the storyline. To unlock the Gold Cup, you must win the Bronze and Silver Cups prior. My tip for beating the Gold Cup easily within the 2 minutes and 5 seconds for the trophy, simply use both shortcuts each lap, I played it safe whilst doing this and still had 15 seconds to spare.
I made this video to help:
Hope this helps.