갔다올게. = I'm going(leaving) now and I'll be back (later). 가다 (go) 's past tense is 갔다 (went) 올게 came from 오다(come). 올게 is like... "I'll come" 갈게 means "I'll go." 갔다오다 is combined word of these two verbs, go and come. 갔다올게 shows the speaker's willingness to go and come for a hearer. I'm going(leaving) now and I'll be back (later).

If that sound was 갔다올께, the more appropriate subtitle is 'I will be right back.' I am leaving now. (저 이제 갑니다.) I will leave first. (저 먼저 갈게요/갑니다.) and plus '다녀오겠습니다(which means I will be right back)' is the most honorific 갔다오다 can be 갔다올게 is casual in spoken Korean. 갔다오다 comes along with ~~게요 like 갔다올게요 which is honorific. However 갔다오다 doesn't come along with 겠습니다 which is the most honorific suffix. In this case, 다녀오다 comes along with 겠습니다 like '다녀오겠습니다.'