A "forged memo" refers to a falsified or fake document, typically a written message or note. It means someone created a memo that appears to be from a particular person or organization, but it is not genuine or authorized. Full definition
If Gleick forged the memo, then he believes that lying to promote an aggressive climate change policy agenda is justified. (fakegate.org)
As for Gleickgate there is simply no proof that Gleick forged the memo. (planet3.org)
GleickGate / FakeGate is about a guy who impersonated someone in a organization he hates, didn't find anything that important, and then went on to forge a memo. (planet3.org)