Use the adjective gradual to describe something that happens slowly, bit by bit, like the gradual increase in the amount of daylight in winter.
Gradual can also apply to the gentle slope of landforms like hills. A gradual incline seems to slowly rise — you may not even realize you're walking up such a hill. Gradual comes from the Latin word gradus, which means "step." Step by step, a gradual hill climbs, while a steep hill seems to leap up into the air.