Given the historical accuracy of the Gallup generic ballot in midterm elections, let's plug this result in to the Swingometer.
In 2008, per Wikipedia, the national popular vote for the House went 53% for the Democrats and 43% for the Republicans. That translates to a 55 D-45 R two party vote when we subtract other votes. That gives the Democrats a 10 point advantage in 2008.
The current Gallup Generic Ballot reads 43 for the Democrats and 49 for the Republicans. That translates to a two party vote of 47 D-53 R, or a Republican advantage of 6.
From R-10 to R+6 adds up to a 16 point swing from 2008. If every district moved 16 points from 2008, the Swingometer projects a 45 seat Republican gain, for 223 seats in the House and the majority.
This will typically be a low end projection in a wave year, additionally, because the "national climate" influences fundraising, voter enthusiasm, retirements, and recruitment.