occtec I can't be entirely certain, but while I have my issues with aspects of Google's business behavior, I'm not nearly as suspicious of them as are many others. I think they sometimes play games, but I don't think they're sinister liars, either.
No company is perfect, but Google has given us a pretty great open source OS and a lot of free utilities to go along with it, many of which allow a decent degree of privacy options, even if the defaults are not what I personally want. I think most of Google's privacy violations are pretty much out in the open. There's too much on the line for them to be too dishonest and sneaky.
Edited to add: if network access is off, the only way to sneak data back to Google is through IPC. If you don't have GMS installed, one of your other apps would need to have some sneaky partnership with gBoard. That just seems tinfoil hat territory to me, but YMMV.