Even though incomplete returns showed Kahn leading his opponent, he conceded the race based on his own campaign's estimate that he would lose.
Gingrey, a physician and an Augusta native, served in the Georgia Senate from 1999 until earlier this year.
He told supporters that President Bush called him to congratulate him on his victory.
"This guy is a wonderful president. We have had a great Republican victory in Georgia," Gingrey said.
The 11th District, which covers 17 counties in west Georgia, had been redrawn by a Democratically controlled Georgia Legislature in an effort to keep the seat in the hands of the Democratic Party.
Kahn, who has never held public office, blamed his defeat on personal attack ads funded by the national Republican Party. However, both sides ran vicious television advertisements.
"It's a shame that personal attacks can do better than discussion of issues," he told supporters.