As a data point, I was employed with a Google competitor a few years ago. We had nowhere near the market share of Google of course, still we did bend over backward to make sure we weren't favoring our own verticals over G's in our results.
They are trying to find what people want to see when they search. If most people who search "email" want Gmail, then it should be at the top.
It's going to be almost impossible to prove that they aren't just better than the competitors at finding what people want vs manipulating search results in their favor.
But if such an intent exists, it is very likely that an email trail would prove it and it might surface during disclosure in a trial.