SWT (Standard Widget Toolkit) is a cross-platform GUI toolkit that ensures a native look and feel on every platform it runs on:

• Glimmer DSL for SWT (formerly Glimmer)

JRuby frameworks that provide alternatives to Java's imperative syntax and FXML's XML-heavy syntax for JavaFX:

• Glimmer DSL for JFX
• JRubyFX

JRuby tools to make the UI development with Swing a more Ruby-like experience:

• Glimmer DSL for Swing
• Cheri::Swing
• Limelight (Project missing at that URL)
• Monkeybars
• Profligacy (with markup language LEL)
• RSwing
• Rubeus
• Swiby (Wayback Machine link to May, 2015)

Use Qt Jambi with JRuby.

• Qt::JRuby