It's nice to see some more font tools! I added a reference to your tool in my thread https://devforum.play.date/t/alternative-bitmap-font-tools/399
Because I am not a regular ruby user, I also needed:
$ gem install bundler
Then I could do:
$ bundle install
Which installed chunky_png OK.
But at this point I get an error I do not know how to fix:
$ bundle exec bdf2fnt.rb -o output courR12.bdf
bundler: command not found: bdf2fnt.rb
Install missing gem executables with `bundle install`