To use any executable it needs to be on your path (unless you give the full path, and that is hardly convenient). For this purpose it is important to know where gems get installed on your system. You should prefer to install gems locally (ie in your home directory) eg /home/tux/.gem/ruby/2.5.0 in which case the path to binaries will be /home/tux/.gem/ruby/2.5.0/bin so then in your configuration file you want to add this to your PATH environmental variable.

~/.bashrc archlinux (and some other linux)

or ~/.profile (mint linux etc)

export GEM_HOME="/home/tux/.gem/ruby/2.5.0"
export GEM_PATH="/home/tux/.gem/ruby/2.5.0"
export PATH="${PATH}:${GEM_PATH}/bin"

For MacOS it is very similar just edit ~/.bash_profile if required

Windows has a gui that allows you to add to the PATH environmental variable (if required)

The executable for JRubyArt is k9 but generally k9 -r my_sketch.rb or k9 -w my_sketch.rb to run watch sketches. Since we are running a two stage process with JRubyArt you can start the process with MRI ruby, but the sketch gets run by jruby directly or via java and jruby-complete.