The valve cover gaskets and the intake gaskets (I think the oil pan gaskets too, but I'm not sure) are not traditional gaskets and don't need to be changed as such. They are actually orange ribbon seals made from Viton (the same material that fuel treated O-rings are made of) and they rarely go bad on the 3800s. You say that you have 106K miles on the engine. I would almost consider thaat low mileage for this engine The Buick 3800 V6 engines have a very good reputation and rank as one of the most reliable engines ever built in automotive history. The engine will probably go more than 300K miles, and outlast the rest of the car with good, basic routine maintenance (oil changes, coolant flush, air filter, PCV valve, serpentine belt, spark plugs & wires, etc.) You said you have a leaking head gasket. I can honestly say that yours would be the first occasion of a 3800 with a bad head gasket that I've ever heard. I'm not saying that it is impossible, but I do find it hard to believe. If you haven't already, I would double check make sure that the oil leak is in fact from the head gasket and not the lower intake which is very possible.
You'll probably never need to change the timing chain. It is durable and it can't jump a tooth because of its design.
The only serious and common problem that I'm aware of with the 3800 Series II V6 engine is the composite upper intake. It has been known to warp as a result of the hot EGR gases that flow through it. This makes a nearby a water jacket weak and eventually it breaks open and causes an internal (or much less often external) coolant leak. Antifreeze gets sucked into the engine which of course can lead to serious internal engine damage in a worst case scenario if is not addressed promptly. A stronger, reinforced replacement composite upper intake is available from AutoZone for around $130. They keep them in stock and it is Dorman part # 615-180. It includes the ribbon gaskets and parts associated with it as well. Click on the link below for more info and a pic of this product.
http://www.pureperformancefever.com/...cbcd10ecdd8fba