What exactly does it mean to *endogenise* a variable in making GDP projections?

In general, an endogenous variable is a variable whose value is determined within the model, while an exogenous variable is a variable whose value is determined outside of the model. It sounds to me as if you are asking about a model in which GDP (the endogenous or dependent variable) is a function of the exogenous (or independent) variables population, human capital, and productivity.