actually 갔어요 and 가셨어요 have same meanings. but,갔어요 is used between friends or people who are younger than you.（朋友之间） and 가셨어요 is used when you say someone who is older than you has gone.(表示尊敬) for example, 나는 수업이 끝나자 마자 집에 갔어요.(我一上完课就回家了) 선생님은 수업이 끝나자 마자 집에 가셨어요.（老师一讲完课就回家了） 我的英语不是特别好，我看你的头像就知道了你是广东的。 所以我用中文来说明了。

I actually had the same question too! Although the explanation I got for this was a bit different. I heard that 갔어요 was just for something done in the past but you continued doing. However, 가셨어요 was like for something done in the past however, is something you no longer do anymore-- or something like that at least. I'm still quite confused on its true meaning. lol