isn't subject to gerrymandering

That's not really true. The majority of states don't do a total tally of votes.

Instead, each district is allocated a point value based on population.

Some states also do a "winner take all" method for each district.

So, if a district votes 51% Republican, the entire district's points is assigned to the Republican candidate.

By making high density blue districts, it allows them to have more slight majority red districts. This means the Republican candidate can win the state even when they did not receive the majority of votes.

This is why state legislature and governor elections are the most important. They control the rules that are used to elect the federal positions.