People will vote with their feet. Most of they brands have said that they withdrew their advertising because they didn't know that their ads were being shown on this channel. For a brand like OVO, Ikea, whatever, they are not going to want to be associated with another brand that their customers think is anti their own beliefs and standards. That's just simple business.
So, other brands will no doubt choose to be associated with the new channel. I haven't watched it long enough to see any ads, but I am sure that there are brands whose brand values match very strongly with the GB News ethos. Or who don't care.