Much of that misinformation is also made with far cruder tools than a well-trained GAN. People will be fooled by what they want to be fooled by. Remember how Republicans thought Nancy Pelosi showed up drunk to some political event because someone just slowed down the video? Or the whole wave of cleverly-edited "expose" videos on ACORN and Planned Parenthood done by the Project Veritas guys? The reality is, anyone who believed those videos already had a negative opinion of those organizations and people that jived with the lie they believed.

It doesn't matter if Nancy Pelosi wasn't actually drunk, she's a lunatic anyway!

It doesn't matter if Planned Parenthood wasn't actually selling body parts for profit, they're baby killers anyway!

It doesn't matter if ACORN wasn't actually telling pimps how to file taxes, they're Obama lovers anyway!

The problem is not that it's possible to convincingly falsify video evidence. That's the kind of problem you need to deal with as the judge of a courtroom, and courts already know how to deal with manipulated and falsified evidence in other contexts: strong monetary and criminal penalties. However, we're talking about the people at large, who are not unimpassioned triers of fact but people who have strong opinions that they are angry about. The court of public opinion is not an impartial or fair court, it is a cacophony of angry people shouting into the void until society gives them a perceived win.

Look at QAnon nonsense: 4chan convinced a good chunk of conservative America that Trump is fighting a global child cannibalism ring by just writing a bunch of incomprehensibly long-winded stories. People aren't stupid, they're angry.