Ikea, cider firm Kopparberg and Octopus Energy have all withdrawn their adverts on the network following its launch on Sunday.
GB News, positioned as a rival to the news and current affairs offerings of the likes of BBC and Sky, is fronted by veteran broadcaster Andrew Neil.
He said the channel would not be “another echo chamber for the metropolitan mindset that already dominates so much of the media”.
However, GB News has faced accusations it will be broadcasting US-style partisan news shows in the UK and campaign group Stop Funding Hate challenged advertisers on social media.
Swedish furniture giant Ikea said it had “not knowingly” advertised on GB News.
It added: “We are in the process of investigating how this may have occurred to ensure it won’t happen again in future, and have suspended paid display advertising in the meantime.”
In its statement, Kopparberg also said it was unaware its adverts were running on the channel and said they had been suspended “pending further review of its content”.
Octopus said it would only advertise with GB News if it proved to be “genuinely balanced”.
The Open University also paused its advertising.
Neil, responding to Ikea, shared a story about the company’s French arm being fined £1 million and its chief executive handed a two-year suspended sentence after it spied on employees for three years.
Neil wrote: “IKEA has decided to boycott GB News because of our alleged values. Here are IKEA’s values – a French CEO who is a criminal with a two year suspended jail sentence for spying on staff.”
Piers Morgan, who has been linked to GB News following his departure from Good Morning Britain, also responded to Ikea, saying: “Oh shut up, you pathetic virtue-signalling twerps. I’m now boycotting IKEA.”
GB News captured a host of high-profile signings, including former Sky Sports presenter Kirsty Gallacher, ex-BBC presenter Simon McCoy and former ITV presenter Alastair Stewart.
In his opening monologue, former BBC political interviewer Neil said the channel will “puncture the pomposity of our elites in politics, business, media and academia and expose the growing promotion of cancel culture for the threat to free speech and democracy that it is”.
GB News will broadcast seven days a week across the UK and Ireland.
Additional reporting by PA