GPSI (pronounced "gypsy") provides computational scientists with a general purpose workbench for developing, testing, and using complex workflows for simulation and analysis. A key aspect that differentiates GPSI from other environments that support large parallel workflows is that it's not tied to any particular science domain. GPSI provides tools that have emerged as common in many of our past science gateway efforts, doing the heavy lifting that is often required in starting a new science gateway, while facilitating customization to a particular domain or collaboration. The GPSI environment integrates support for job execution and management; data management and browsing; and application development and reuse. We are working with research groups in power grid simulation and analysis and in phylogenetics to improve productivity via GPSI.