This project is built using Gosu, available as a ruby gem. Gosu is built on top of SDL2, a C library, so you will need this installed too. Please see Gosu's wiki for how to install Gosu's dependencies.
Please see the Gemfile for the gems this directly depends on.
I recommend installing those gems using Bundler
If you have used bundler, then execute bundle exec ruby snek_game.rb, otherwise ruby snek_game.rb.
Tested with ruby 3.0.3p157 (2021-11-24 revision 3fb7d2cadc) [arm64-darwin21]