The culture secretary warned that basic democratic values can no longer be taken for granted after the upstart channel saw the withdrawal of advertising from companies such as Ikea, cider firm Kopparberg and Octopus Energy.
The network, which launched last Sunday, has promised to take on so-called cancel culture.
Writing in a column posted in the Sunday Telegraph and the Gov.uk website, Dowden said: “When he launched the channel, veteran broadcaster Andrew Neil vowed that GB News would not be ‘an echo chamber for the metropolitan mindset’, and that it would ’empower those who feel their concerns have been unheard’.
“Rightly so. A free media is one that has a diverse range of opinions and voices – and as I said earlier this week, GB News is a welcome addition to that diversity. We need outlets and commentators who cover the range of the political spectrum; who can speak truth to power; and who are willing to challenge dogma or orthodoxy.”
Dowden continued: “I’ve no doubt plenty of people will disagree with some of the things GB News commentators have to say - just as plenty of people disagree with the things they see and hear on the BBC, Sky News or any other media outlet. But if you don’t like those ideas, switch over - don’t silence. We shouldn’t be blocking people from the conversation simply because we disagree with them.”
He added: “Across the West, our values of tolerance and freedom of expression increasingly risk being undermined by a small but vocal minority.
“For them, these are not absolute, but relative, concepts, ready to be bent to silence dissent from their world view. We will not stand by and allow that to happen.”
GB News has faced accusations it will be broadcasting US-style partisan news shows in the UK with campaign group Stop Funding Hate challenging brands not to place ads on the channel before it had even launched.
While GB News has repeatedly insisted it is committed to producing impartial journalism, this week saw a number of brands suspend their advertising on the network.
Swedish furniture giant Ikea said it had “not knowingly” advertised on GB News and said it was “in the process of investigating how this may have occurred to ensure it won’t happen again in future”, pulling their ads in the meantime – something GB News chair Andrew Neil hit back at on Twitter.
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”.
Energy company Octopus said it would only advertise with GB News if it proved to be “genuinely balanced”, while The Open University also paused its advertising.