ReAction GUI it is the name of the widget toolkit engine that is used in AmigaOS 3.5-4.1. It is an evolution of ClassACT, which is an object oriented system of classes that enhanced the aspect of the Workbench 2.0 GUI of AmigaOS. The native Amiga windowing system is called Intuition, which manages the rendering of screens, windows and basic widgets. However, until AmigaOS 2.0 there was no standardized look and feel, and often application developers had to write their own non-standard widgets (both buttons and menus), with Intuition providing little support.
Intuition was later enhanced with gadtools.library (AmigaOS 2.0 and later), which provides a set of standard widgets. AmigaOS 2.0 also introduced a standard dialog system called ASL (Amiga Standard requester Library) and the Amiga User Interface Style Guide, which defined how applications should be laid out for consistency. (Full article...)