While the oil pan and timing chain cover are separate, disturbing the lower engine components to access the timing chain often necessitates at least partially removing the oil pan. Even if you manage to avoid completely removing it, the risk of damaging the existing gasket during the process is significant. A damaged or old oil pan gasket is likely to leak, requiring further repairs down the road.
Therefore, while not strictly required, replacing the oil pan gasket while you have the area open is a wise preventative measure. The cost of the gasket is relatively low compared to the labor involved in getting everything back together. It saves you time and potential future headaches.
