Both 얇게 썰다 and 가늘게 썰다 have same meaning. Strictly, 얇게 썰다 is more natural for for slicing plain think object 가늘게 썰다 is more natural for slicing round and long object 가늘다 is used when the girth of an object is thin. 가늘다 is used for things that are generally round and long, like tree trunks, legs, and pillars. For example, 팔뚝·허리·종아리 얇다 is used when the thickness of an object is thin 얇다 is used for things that are more spread out, like paper and pizza dough. like For example, 종이·널빤지·책·판자