Pretty much the entire history of progressivism is a long, slow dawning realization that it's not harmless to treat individuals on the basis of group statistics, for one group category after another. Part of the problem is feedback loops - you can inadvertently amplify subtle perturbations by feeding them back to people. For example, it might seem harmless to subtly nudge a woman away from a career in STEM, on the basis that women are rare in STEM and therefore they probably won't enjoy it. But this action, repeated across millions of people, creates the pattern that you are reacting to. I think the principle probably generalizes to "everyone", not just "women" and "black people".
In the case of autocomplete suggestions, they can still cause harm even when they're statistically likely. What do you think Google would autosuggest for "Blacks are...", if it were around in 1950? What would the statistics on the completion of that sentence look like? And would the effect of someone seeing that list be 100% neutral - or would it subtly nudge them?
There's nothing obviously harmful with the specifics of GMail's auto-suggestions now. But the principle of 'we offer the suggestions that you've shown us you're most likely to want to use' is not morally neutral.