UI test automation is the top layer of test automation pyramid, and one of the most important automation to achieve continuous delivery process, where functional tests simulate real user interaction with the application and validate the system output/results.
Test automation for Web Applications has become a W3C standard through WebDriver Protocol, which means that each new browser version should provide that remote control interface.
When working on GeneXus, programmers don't have full control over HTML elements (id's, XPath, etc) since GeneXus take care about how to generate source code. That's why it is so important to have an automation framework that takes care of underlying technology used and provide an abstraction layer capable of making test automation easier and maintainable with development.
UI Test automation in GeneXus is achieved using GXtest.
Official GXtest 3.x documentation is provided here.
GXtest v4 (in beta process) documentation is provided in this Wiki. Please take a look at the roadmap here.