I know it might sound unnatural but I'd like to know which one is correct in terms of grammatical structure...
- I work in Google's office
- I work at Google's office
- I work in Google office
- I work at Google office
(1) and (2) are grammatically correct, but sound unnatural to me. I would prefer to write "I work [in/at] the Google office." But you might well see native speakers write a possessive like that; I just think it's clunky. On the other hand, if you wrote "Google's Mountain View office," it would sound fine to me, so I'm not sure that this distinction actually makes a difference.
(3) and (4) are ungrammatical because they lack a determiner before "Google office."
In general, "at the office" is used to refer to the workplace as a whole (by analogy to "at work"), while "in the office" might be interpreted as a specific room or cubicle, but it could also refer to an entire building. You can be at the office even if you are in the parking lot, but if you are in the office, then you are physically inside. Since it's rarely necessary or useful to specify that one is physically inside, "at the office" is more idiomatic in most contexts. (For example, if you say "I work at the office," nobody is going to imagine that you work in the parking lot unless there's some other context indicating as much.)