When you know marketing, you understand most marketing advice online is bullshit. When you’re an expert in statistics, you see news gets the numbers twisted. If you're a professional football coach, it pains you to hear people at the bar talk about football - such are the inaccuracies in their discussion.
Then you read the next thing in the newspaper or Reddit or Twitter - it’s a topic you’re not an expert in - and you blindly accept the information as fact.
So Gell-Mann amnesia is a tendency to forget how unreliable the news - or any information - is. We're reminded of the inaccuracy whenever there's a topic we know a lot about, yet we're not critical enough when we face a less familiar topic.