Techniques available in the literature to enhance the robustness of generic model control (GMC) are reviewed. We show that none of these techniques utilize a rigorous analysis of the process-model mismatch. This paper presents GMC from the geometric perspective in order to analyze the effect of process uncertainties in the closed-loop response. The geometric approach defines a performance surface for the closed-loop response with no model error. When the model is not perfect, the system leaves the surface, making the response different from the reference. A technique based on sliding mode control, called sliding generic model control, is developed here to take into account the process uncertainties and to keep the system close to the GMC performance surface.