V + 는 김에 roughly translates as ‘while you’re at it’ or ‘since you’re already doing V’. Unlike (으)면서 (which means ‘while V-ing‘), this structure suggests it is more convenient to do the second action because you are already doing the first. ㄴ 김에 is used in the past tense and 는 김에 in the present/future tense:
서울에 온 김에 한강공원에 갔어요.
As I was in Seoul, I went to Hangang Park.
시작한 김에 끝까지 할 게요.
Seeing as I’ve started, I will finish.
샐러드를 만든 김에 야채수프도 만들었어요.
While I was making salad, I (took the opportunity and) made vegetable soup as well.
말이 나온 김에 여행예약을 하자.
Speaking of which/while we’re talking about it, let’s (take this opportunity to) make our travel reservations.
은행에 있는 김에 가스비를 지불해야 돼요.
As you’re at the bank, you should pay the gas bill.
서울을 지니가는 김에 동대문시장에 가봐야 돼요.
As you’re passing through Seoul you should visit Dongdaemun Market.
형민에게 전화를 하는 김에 내일은 축구를 하고 싶은지 물어봐 주실래요?
Since you’re going to call Hyeongmin, can you ask him whether he wants to play football tomorrow?
약국에 가는 김에 치약을 사 주세요.
Since you’re going to the chemist, can you please buy me some toothpaste?
겨울 방학 동안 영국에 오는 김에 식사를 하러 런던에서 같이 만나자.
As you’re coming to the UK during the winter holidays, let’s meet up in London for dinner.