And since it’s GPL that any additions are compatible with the GPL, which the ad / tracking stuff they’re likely to add likely isn’t.
That's a good point, although I wonder if there are any ad SDKs that are GPL compatible? There's no reason that couldn't exist AFAIK.
However, there's also the much simpler scenario where they straight up replace the apps with something completely different. This company buys apps all the time, so I'm sure they have at least a few calendar, gallery, file browser, etc apps lying around that they can reuse.
Please Excuse My Dear Aunt Sally, she downloaded a shitty ad-infested calculator from the Google Play store.