With gerrymandering, the ruling party will pack voters in districts by party. Instead of competitive districts, one party districts are formed, and more extreme candidates win primaries. That's how you end up with really objectionable people being elected instead of centrists, and compromise becomes near impossible.
Democrats won 50.8% of the House popular vote, and 51% of seats.
Or in 2018: Democrats won 54% of seats and 53.4% of the vote.
It had a modest effect in 2016: Democrats won 45% of seats but 48% of the vote.