The programme, which began broadcasting yesterday, 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 last year in the wake of her appearances on last year's Strictly Come Dancing.
Reid - who wore a dazzling red dress for launch day - has been joined on the weekday ITV show by Ben Shephard, Charlotte Hawkins and Sean Fletcher.
For viewers, one of the biggest changes to predecessor Daybreak - along with the presenting team - was the introduction of a glass desk at which the hosts were seated for much of the show, although they moved to sofas later in the show.
Many observers have interpreted the discussions around the desk as more of a US-style approach, taking its cue from ABC's popular Good Morning America.
Viewers were divided on the show, with some lamenting the passing of its predecessor while others praised its fast pace and gloss.
The programme also featured Andi Peters presenting a regular "Wheel Of Cash" competition slot from a different location each day, inviting the public to win prizes.
Yesterday's Good Morning Britain launch peaked at 1.2 million viewers.
Helen Warner, ITV's director of daytime, was pleased with the performance and said: "The launch was a fantastic team effort, and we are focused on building a strong breakfast brand going forward."