What ended up working for me after a few hours of pain..
if you're running brew..
brew install ruby
in the terminal output/log, identify the path where ruby was installed, brew suggests 'You may want to add this to your PATH', so that's what we'll do. For example, mine is
/usr/local/lib/ruby/gems/3.0.0/bin
Add this to your path by running (omitting braces)
echo 'export PATH"{the_path_you_found_above}:$PATH"' >> ~/.bash_profile
then update your environment by running
source ~/.bash_profile
now, try running your install, i.e.,
sudo gem install middleman