You're correct that at the first order gerrymandering is irrelevant to the Senate and Presidency. Gerrymandering comes into play as a second-order effect, though; when used at the state level, it can cause state legislatures to effectively lock to a party. These state legislatures can then make changes to the way voting is organized in their state, enfranchising and disenfranchising voters in ways that support their party, and funding/defunding polling districts per the same. This has an effect that can be plenty large enough to swing statewide elections: see the last election's shenanigans in South Carolina, Georgia, and Florida if you're in doubt.