Grid simulation tools provide frameworks for simulating application scheduling in various Grid infrastructures. However, while experimenting with many existing tools, we have encountered two main shortcomings: (i) there are no tools for generating workloads, resources and events; (ii) it is difficult and time consuming to model different Grid levels, i.e. resource brokers, and local level scheduling systems. In this paper we present the Grid Scheduling Simulator (GSSIM), a framework that addresses these shortcomings and provides an easy-to-use Grid scheduling framework for enabling simulations of a wide range of scheduling algorithms in multi-level, heterogeneous Grid infrastructures. In order to foster more collaboration in the community at large, GSSIM is complemented with a portal (http://www.gssim.org) that provides a repository of Grid scheduling algorithms, synthetic workloads and benchmarks for use with GSSIM.