What if the attacker instead advertises `[email protected]`? How are you even supposed to know who wrote the legitimate version of the package in the first place? And if you _do_ know who wrote the package, you don't need a GPG key to verify that; just their NPM username or even the actual, real package name will do fine for preventing this specific attack.
Websites, Twitter, Github, Keybase, etc..
It would be pretty hard for a bad actor to overtake the real author's entire Google-findable presence (assuming it's a reasonably popular package - why would you typosquat anything obscure).
If all you do is send an email, then you haven't really done "due diligence" in any acceptable form.