GUI testing is essential to provide validity and quality of system response, but applying it to a development is not straightforward: it is time consuming, requires specialized personnel, and involves complex activities that sometimes are implemented manually. GUI testing tools help supporting these processes. However, integrating them into software projects may be troublesome, mainly due to the diversity of GUI platforms and operating systems in use. This work presents the design and implementation of Open HMI Tester (OHT), an application framework for the automation of testing processes based on GUI introspection. It is cross-platform, and provides an adaptable design aimed at supporting major event-based GUI platforms. It can also be integrated into ongoing and legacy developments using dynamic library preloading. OHT provides a robust and extensible basis to implement GUI testing tools. A capture and replay approach has been implemented as proof of concept. Introspection is used to capture essential GUI and interaction data. It is used also to simulate real human interaction in order to increase robustness and tolerance to changes between testing iterations. OHT is being actively developed by the Open-source Community and, as shown in this paper, it is ready to be used in current software projects.