TBH, getting an Office file opened from Google Drive into Office iOS apps is easy:
Google Drive/folder/file/tap/open in, etc - voila!

Not really that much different from opening a file from any directory/folder on a PC/Mac. However, getting a file
from 0ffice onto Google Drive is another issue.
Odd, if Android/iOS wars are so pertinent to this issue, that so many other iOS apps allow direct export to Google Drive??? In fact, this is precisely why Myself, 100 colleagues and 1250 students at my school, are currently using Pages, Keynote and Numbers,
as productivity apps on iOS, when we'd much rather use Office!
Clearly an omission on the part of Google/Alphabet, or whatever they're called these days. Perhaps their disintegration into smaller, more focussed companies, will preclude such oversights in future. 😆
Anyhow, accepting that this is an unresolvable issue from the Google end, perhaps MS could take a look at allowing Google Drive to import files from OneDrive for Business (works great in OneDrive, so I'm guessing this is problem is with SharePoint, i.e.
at MS's end). At least then my colleagues, students and I could use this back door to get Office files where we actually want them!