> GDS’s GA Premium account contractually prevents Google from investigating the data

What's the penalty for completely disregarding that?

Is it anywhere near large enough to justify them not disregarding it?

More to the point: Pretty much the only thing a contract can do is impose a monetary penalty. Even if it's quite large, it's still only money, and Google rakes in money by the bushel basket from selling data to advertisers. Therefore, it isn't a very convincing penalty.