Prod

Expressions can contain a function that calculates the product of set indexed expressions or terms (function.gms).

X=prod(I, a(i)*0.2-0.5);

Eq7..   z=e= prod(I, a(i)*0.2-0.5);

This function is continuous and smooth.  It can be used on data during GAMS calculation or in models.  It's use in .. equations in terms that involve variables requires the model type be NLP or one of the other types that handles nonlinear functions  More on prod can be found here.