The show, which features former BBC star Susanna Reid, drew just over half of the audience of her old colleagues, according to overnight figures for its launch.
Its average audience of 800,000 was also down on the first-morning figure for its predecessor, Daybreak. That show had one million viewers when it began in 2010.
Mr Fincham said the show was fresh and different.
He added: "This is two or three weeks into the life of Good Morning Britain. In the life of a breakfast show that is a nanosecond, so it's very early days and we're fully right behind it"..