I don't think anyone has argued that GB news should not be allowed to air, not has anyone campaigned to prevent it airing.
What they have done is ridiculed and criticised it. Is that not allowed in your 'cancel culture' narrative?
If advertisers want to restrict their advertising to ethical media outlets to avoid any negative feedback, they're perfectly entitled to do that, it's their money they're spending, their products and services they're promoting, unless that also is not allowed in your 'cancel culture' narrative.
https://stopfundinghate.info/2021/06/14/gb-news-advertisers/
I think trying to get advertisers to pull their funding from GB news is campaigning to prevent it from operating.
Again, they started their campaign before GB news aired.