too slow to respond...damn
you say its a gst...that means its turbo...easiest way to tell is look for the turbo...it'll be right infront of the valve cover under the exhaust manifold
here are your motor options:
420a
4g63:
however i do believe that in 99 they had a 4g63 motor that was N/T...so like i said...look for the turbo between the radiator and block below the exhaust manifold