They're actually not focusing on gun channels. It's a long story, but I'll try and make it short.
Over the last few years, old media (TV, newspaper, radio, HBO etc.) has been losing massively to new media (social media, Youtube, Netflix). This caused sort of a "war" in the industry.
One particular journalist from the Wall Street Journal decided to fire a shot, directly at the Youtuber with the highest subscriber count, Pewdiepie. He dug up a video where Pewdiepie made some Nazi jokes and another where he paid a guy $5 to hold up a sign saying Hitler Did Nothing Wrong as a (very distasteful) joke. He then followed up this salvo with some screenshots of hate videos for things like white supremacy or ISIS, next to ads for Pepsi and H&M; The reported alleged that Youtube was not doing enough to control where ads appear and thus is not a desirable place for advertisers.
It later turned out that this same reporter was in the process of trying to patent software related to controlling what content ads appear on. He was also likely just jealous big brands are advertising on Youtube instead of his dying paper. It also turned out some of his screenshots were fake, and that he had made the same sort of jokes about Jews and African-Americans on his Twitter. None of this matters because people only read headlines.
Thus begins the "ad-pocalypse". Companies, fearing a PR nightmare of having their logo next to hate speech, as well as probably wanting to take any chance they can to pay less for advertisements, start pressuring Youtube to remove their ads.
The reality is that Youtube can't "smartly" filter for this. 300 hours of video are uploaded every minute. Yes you read that correctly. All they can do is make software, specifically "the algorithm" to try and control content. The design of this software is secretive for obvious reasons, it's like their 11 secret herbs and spices.
Some people speculate, that it controls content based on machine-learning from several selected thumbnails from the video, snippets of audio content, keywords in the title and description, or a combination of all that. So it's wrapped a lot of channels up in getting demonetized. For example, it can conflate a video about not bullying LGBT teens with sexual content. It can stop a video about ending animal abuse because it contains animal abuse, etc. It makes no sense and they're likely working on it. But this is pretty cutting edge technology and I wouldn't expect a fix overnight.