Thermostat..dumb question
 

98 Silverado with 350 Vortec. Here's a dumb question for all, and maybe I'm completely overthinking this. I recently had my oil changed and the attendant told me I needed to change my thermostat because there was antifreeze just below where the thermostat sits. So I went to an O'Reilly's to buy a thermostat and replace it myself (the quick lube place wanted $65). When replacing the thermostat, is there a gasket that must be replaced too?? When I asked the O'Reilly's person about the gasket he told me they don't even sell them.

He should have recommended you get the gasket when you bought the t-stat. But yes there is either a gasket, or a rubber o-ring like mine has in my 2001 5.3