Each feature in the production build of GMAT has an engineering specification that defines the working requirements, the user interfaces, and how the feature works. GMAT's features are broken down into several categories to help organize and track related features. Functional specifications are maintained in Google docs primarily because Google docs supports comments and markup during the review phases better than the wiki. Features in GMAT are tracked by requirement Id. For example, the Spacecraft Orbit State feature is governed by the requirement FRR-1.
Links to GMAT's feature specifications are below.
Basic Dynamics and Models Specifications
Powered Flight Specifications
Solver Infrastructure Specifications
Programming Infrastructure Specifications
Output and Utility Specifications
Application Control Specifications