You can use Ruby-Processing in two ways.

Use the jruby-complete.jar that Ruby-Processing ships, the Gems-in-a-Jar approach. In this mode, all gems you install will be packaged as part of the JAR.

Using the JRuby already installed on your system.

If you’re following the first approach, first head to the location where the jruby-complete.jar is located, for Ruby-Processing. There, do this:

java -jar jruby-complete.jar -S gem install basis-processing --user-install

Alternatively, if you’re using a conventional JRuby installation, do this:

sudo jruby -S gem install basis-processing