Why replace the water pump? You should be able to check that by taking the cap off (when cold) and seeing if the coolant enters the engine block when the tstat. opens. Either way if it was an underperforming waterpump, you would notice the problem when driving since you would overheat.
Gaskets? Why? You dont leak gallons through gaskets? Do you know where it is leaking from? To dump a gallon when stopped there are only a few places I can think. Petcock valve, Radiator overflow, and cap.
Recommendations: Check cap pressure, replace (cheap, and worth a shot), I am not familiar with this engine, but if it has an electric fan, check if that is functioning. Leak in radiator/hose is also a possibility. It also sounds like the system is not holding pressure (lowering booiling temp).
There is also the possibility the engine cooling system is clogged or restricted. Did you try a reverse flush?
Hopefully this will give you some ideas. Without exact specifics, it is a tough one to diagnose. I am also making an assumption here that you are having a boilover condition. If it is just a leak, chances are it is leaking all the time. In this scenrio I will check for leaks and if I see any coolant, make a note of it and the steam clean the engine (when it has not been running). Then if I can duplicate the scenrio, I will get a fluid trail or a leak point.
I just had this same type of scenrio on a 95 ford explorer, I had fluid on the block and knew it was leaking from the manifold gasket, just couldnt see where. I now have pinpointed the leak and will replace the gasket.
Feel free to contact me at firstname.lastname@example.org.