Yes, his handling of the situation in Gaza is the main reason for my disapproval of him. As for why that weighs so strongly in my assessment of Biden, it's because I believe that it is absolutely imperative to give a lot of weight to tens of thousands of deaths and more than twice as many injuries (38,000+ deaths and 88,000+ injuries, so far).
I use various sources to stay updated on US policies, as I said, including (but not limited to) official US government websites, the Associated Press, and Reuters. As far as sources with different leanings and opinion pieces that give perspectives from specific viewpoints—including ones I especially oppose—I check out many other sources including (but again not limited to) the Hill, the New York Times, the Guardian, France24, the Times of Israel, Al Jazeera (in English), and others that range from highly reputable to so opinionated as to be editorial. I don't give all sources equal trust in terms of reliability, but I prefer and try to be aware of as many different perspectives and analyses, including opinionated ones, as I can.
I could go into more details about my opinion of Biden, which is based on his policies rather than the position of any one media outlet, but I don't want to derail this thread. I hope this post clarifies at least the nutshell version of why I have the viewpoint that I do, though.