as far as I can tell from my own investigations into it, Murray Gell-Mann did not coin this term or have much of anything to do with it. Murray Gell-Mann himself was a quantum physicist and I don't think he spent a lot of time worrying about the news media.
I believe the "Gell-Mann Amnesia Effect" is something created by the fiction author Michael Crichton as a plot device. It may be an interesting idea but as far as I know it is not a real researched effect in media studies or psychology.