You can use "-ㄹ 겁니다" or "-ㄹ 것입니다" for the formal form of it.:) e.g. I will do it = 그것을 할 거예요. = 그것을 할 것입니다. = 그것을 할 겁니다. As for "거입니다", "거" is the spoken form of "것". -ㄹ 것입니다 = -ㄹ 거입니다 = -ㄹ 겁니다(거입 => 겁) In terms of grammar, "-ㄹ거입니다" is correct but "-ㄹ것입니다" or "-ㄹ겁니다", which is the contraction of "-ㄹ거입니다" is preferred in everyday Korean. \^o^/

You can use both 것입니다 & 겁니다.