I hope I don't misunderstood your question. Particle "고 있어" is used to describe when something is currently happening (I think it's similar to Present Continuous/Present Progressive Tense). So "가고 있어" means "I'm going.." implying that person is currently on the way. While particle "았/었요" is used to describe when something is already happened (I think it's similar to simple past tense). So "갔어요" means "I went.." implying that person went to the said place sometimes in the past. I hope you can understand my explanation. :)

가고 있어 is a present progressive. I'm going. 갔어요 is a past tense, and also in an honorific form. I went.