To help meet the ever-increasing demand for cloud computing services and resources worldwide, while providing resilience and adequate resource utilization, cloud service providers have opted to distribute their data centers around the world. This trend has been motivating research from the data center management research and practitioner community on new job schedulers that take into account data center geographical distribution. However, testing and benchmarking new schedulers for geo-distributed data centers is complicated by the lack of a common, easily extensible experimental platform. To fill this gap, we propose GDSim, an open-source, extensible job scheduling simulation environment for geo-distributed data centers that aims at facilitating the benchmarking of existing and new geo-distributed schedulers by subjecting them to a variety of data center features and conditions We use our geo-distributed job scheduler simulation platform to reproduce experiments and results for recently proposed geo-distributed job schedulers, as well as testing those schedulers under new conditions which can reveal trends that have not been previously uncovered.