I could see Gowron giving the House of Mogh all the Duras holdings after the civil war just as an fu to them. It's never specifically said, but it definintely is implied they owe their power to Gowron's nepotism in some form.
It's also possible Kurn accumulated some land and holdings while he was in the House of Lorgh. Kurn did have a wife and several daughters it was mentioned. With all the "old friends" Mogh seemed to have, it's not impossible they'd all flock to the House's banner either.