The question is how to arrive at a system in which another equation is enforced based on the value of a function. To be fairly general, use the following statement:

   z = f(x) when y > 0
   z = g(x) when y < 0

Note that the distinction between >= and just > is not meaningful for a numerical algorithm on a finite precision machine.

Start by writing, in GAMS,

   Variable fs, gs;
   z =E= (f(x) - fs) + (g(x) - gs);

So, when y > 0, we want fs = 0 and gs = g(x). When y < 0, we want fs = f(x) and gs = 0;

Now declare a binary variable, b, that will be 1 when y > 0 and 0 and y < 0.

   Binary Variable b;
   Positive Variable yp, yn;
   y =E= yp - yn;

   yp =L= ymax * b;
   yn =L= ymax * (1 - b);

Next, split the two terms of z into positive and negative parts.

   Positive Variable fp, fn, gp, gn;
   f(x) - fs =E= fp - fn;
   g(x) - gs =E= gp -gn;

   fp + fn =L= fmax * b;
   gp + gn =L= gmax * (1 - b);

So, b = 0 ⇒ fs = f(x) and b = 1 ⇒ gs = g(x).

Finally split just the f and g slacks fs and gs into positive and negative components.

   Positive Variable fsp, fsn, gsp, gsn;
   fs =E= fsp - fsn;
   gs =E= gsp - gsn;
   fsp + fsn =L= fmax* (1 - b);
   gsp + gsn =L= gmax * b;

So, b= 0 ⇒ gs = 0; and b = 1 ⇒ fs = 0;

Taken together, the last two sections give b = 0 ⇒ z = g(x) and b = 1 ⇒ z = f(x).