갈다 is grinding or milling something down — garlic, coffee, matcha: 녹차를 곱게 갈아 가루 형태로 만든 (green tea ground fine into powder), 마늘을 이렇게 갈아서 넣어요. A separate 갈다 means replacing something, which is where 갈아입다 'change clothes' comes from (서핑복으로 이렇게 갈아입고 왔는데). Beware two lines that are neither: 매콤하게 갈게 and 간단하게 갈게요 are 가다 'go', in the sense of 'I'll go spicy / I'll keep it simple'.