겁니다 is just short for 것 입니다 in korean, you could construct a phrase out of two ways: either by a typical S-O-V or by normalization of that. there are many ways to normalize thing. 는 것 is just one typical way of doing that. By normalization, you are treating the verb (v) like a noun, similar to gerund in english (-ing). Don't ask me why it's done this way. all i know it is a form of normalization - its meaning is not really that much different from a subject-object-verb construction. Yet you will encounter a lot of this sort of normalization in true real-life scenario so just get used to that. There is another usual form of normalization when you leave note for others on a sign or notice. e.g. 알바구함 (seeking for part-time position) here 알바 구하다 is the raw form. By adding ㅁ at the end of a verb you are in fact normalizing the verb form into some sort of a noun. This style is very common on notice and things you leave instructions on a notepad to others to read. Again don't try to ask why it's done this way. Every language has its own way to convey a message. These are just some pecularities that you have to adapt to.

yeah your right if you ask somebody what are you doing in Korean you should say 지금뭐해요? 겁니까 comes from Korean word 걸다 which means to hang 뭐 하시는 겁니까 is a different kinda of what are you doing, like for example people say 여기서 뭐 하시는 겁니까? what are you doing here? ( what are you doing hanging around her?