Thermostat gasket

Hey guys,

I have a question about gaskets. The car is an 89 98, with the Series I 3800 in it. I removed the water outlet on the top of the engine, and I have the new o-ring that will seal that, but the old thermostat also had a small gasket around the top. The new thermostat I have has a larger diamter on the top, the gasket will not fit around it. What is the purpose of this gasket? Seems like it is just used to separate warmer and cooler water in the system. Is it needed?

Thanks,
matt