Is this the thing they call the "Google" app, or a different Google app (I don't use their feed stuff myself).
In any event if it's using 16GB I assume that either this is almost entirely data or your system is misrepresenting its size (the "Google" app on my Pixel has an app size of about 160MB). So even if you could move it to SD (which won't be possible if it's pre-installed as a system app, root or not) it would make very little difference as that does not move the app data - there are tools that do, but really it shouldn't be using 16GB of data.
If you visit it in Settings > Apps and press the "manage data" button (assuming it is the Google app) what does it show for the different types of data usage?