Grant's "powers" (Grant Powers would be a really cool action move hero name) are not powers at all, but are instead simply abilities gained over time. After countless years of climbing up and down pirate ships, keeling hauling and hoisting things, his climbing abilities became so profound that he was able to scale almost any surface.
Honestly, it's not all that uncommon from what I've been told. My uncle was in the navy, and used to love showing off at parties by climbing the walls and ceilings. At least that's what my mom said he was doing right before he fell out of a window to his death. My dad said that he was just seriously high, which makes sense considering the coroner said he had broken every bone in his body. I guess my uncle shouldn't have climbed so high.
Yup, Grants abilities are fairly common actually.