The research topic of digital twins has attracted a large amount of interest over the past decade. However, publicly available exemplars remain scarce. In the interest of open and reproducible science, in this exemplar paper we present a lab-scale gantry crane and its digital twin. The exemplar comprises both the physical and digital side of the twin system. The physical side consists of the physical crane and its controller. The digital side covers the CAD models and kinematic model of the crane, and provides services for optimal control, historical data logging, data visualization and continuous validation. We used this setup as use case in several previous publications where its functionality was validated. It is publicly available and only relies on other freely available and commonly used software, this way we hope it can be used for future research or education on the topic of digital twins.