first, '갈것어' is a wrong word. '-것어' is not used. and '갈겠어' is a wrong too. but its right expression is "가겠어" and it means "(I) will go" '갈게요' means "(I) am going to go" -_- It is same '가겠어' but "가겠어" is more strong feeling. "갈거야?" means "(do you) go?". but "갈거야." is "(I) am going to go". so Did you see "갈게요"and "갈거야." are same? but these words differ who you talk 'to some one' "갈게요" is more poilte than "갈거야." am I good for you?