I had a 88 Bonneville, and now have a 96 Bonneville, both with Pontiac’s 3.8 6-cylinder, fuel injected V-6. The Grand Prix is little smaller, but I think had engine choices similar to the Bonneville. If not, this may not help much.
Both Bonnevilles had/have all solid-state electronic ignition. In other words, no mechanical distributor at all, it’s a completely solid-state module that gets its timing signals from the main computer. So the timing belt (not a chain on that particular engine) has nothing to do with the distributor timing.
However, even though these engines don’t have a mechanical rotor for a distributor, the timing belt drives the camshaft, which must be kept in sync with crankshaft, or your valves open and close at the wrong time. So the belt and gears still need to be replaced at some point.
IIRC, Pontiac recommends replacing the timing belt and gears at around 100,000 miles. I had the one on my 88 replaced at around 165,000, even though it was still running strong. But I was heading out on a 3 thousand mile trip and figured it was time to do it just out of CYA concerns. (On a side note, that car ran another 50,000 before I gave it to my sister, who drove it for 2 ½ more years before she crashed it. It was still running strong and had never had any major engine work.)