How do we classify a nozzle to be grossly overexpanded in the design phase? For example, looking at the dimensions and operating at sea level, can we determine if it will be overexpanded or grossly overexpanded in case of rocket nozzles?
The only technical definition I have seen of "grossly overexpanded" is from here.
Nozzle operation was kept to overexpanded (OX) and grossly overexpanded flow conditions (GOX). In this work, GOX flow was defined as nozzle operation at a pressure ratio lower than that required for flow separation.
The term "grossly overexpanded" is used in the Wikipedia Altitude Compensating Nozzle article. It references Huzel and Huang Design of Liquid Rocket Propellant Engines, but the term does not appear in that work.
(Huzel and Huang, p. 7)