Lots of people have been getting:
ERROR: While executing gem ... (TypeError)
incompatible marshal file format (can't be read)
format version 4.8 required; 60.33 given
when executing gem
But, this stackoverflow got it right. The problem is accurately described by the comment:
I'd like to add some nuance to this answer: ~/.gemrc contained a referce use
rubyforge as a source. This was relevant 10 years ago, but these days gems
live on rubygems.org, so you should have a reference to have
https://rubygems.org as source instead.
I moved .gemrc aside, and it stopped asking for data from gems.github.com, which actually was returning a 404, and the problem went away. ARGH. If only Gem had errored on the URL correctly.