The programme, which began broadcasting on Monday, had an average audience of 800,000 viewers while Reid's old show, BBC Breakfast, pulled in 1.5 million.
The launch of GMB was also markedly down on the first-morning figure for its predecessor, Daybreak, which pulled in one million viewers when it began in 2010. ITV chiefs can draw some comfort from the new show proving more popular than the audience levels of around 600,000 which axed Daybreak had recently been achieving in recent months.
Reid was the big signing to launch the new show after being lured from the BBC, where she had proved to be a popular figure. Her profile increased hugely late in 2013 in the wake of her appearances on last year's Strictly Come Dancing.