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 on the UI and also in the database.
Test automation for Web Applications has become a W3C standard through WebDriver Protocol, which means that all popular browsers are committed to providing this compatibility on every new version.
When working in GeneXus programmers don't have full control over HTML elements (id's, XPath, etc), and ever less control regarding rendering events of the DOM, since GeneXus just take care about it and generates the source code of the app. That's why it is so important to have an automation framework that takes care of underlying technology used and provides an abstraction layer capable of making test automation easier and maintainable along with development.
UI Test automation in GeneXus is achieved using GXtest.
GXtest v4 documentation is provided in this Wiki.