The Gamma Statechart Composition Framework is an integrated tool to support the design, verification and validation as well as code generation for component-based reactive systems where the behavior of each component is captured by a statechart, while assembling the system from components is driven by a domain-specific composition language. Gamma automatically synthesizes executable Java code on top of existing code generators, and it supports formal verification by mapping composite statecharts to a back-end model checker. Execution traces obtained as witnesses during verification are back-annotated as test cases to replay an error trace or to validate external code generators.