GSC : an inspection by a suitably qualified person of a domestic appliance using gas as a primary fuel, to establish that the former is operating correctly
EPC : an inspection of the external fabric of a domestic dwelling to evaluate its current ability to hold the heat in, and comment on future potential to improve its thermal performance.
My definitions, admittedly, but sounds like two very different things to me. Then again, I do aim to set myself up here as a supplier of high quality and accurate answers - apart from 17th Electrical Regs questions where I bow to BS's greater understanding. BM