I started using the term "gray code" while working on SBIRS High. We were asked to restructure our codebase to seperate the application-specific parts from the infrastructure so that the infrastructure could be reused in another computer within the same satellite (where many of the general command and telemetry paradigms were the same). Inevitably we ended up with some files that contained both application code and infrastructure code (they were neither black nor white) -- gray code.