it's not a gasket, it's a small needle with a rubber tip that connects to the floats.  They usually come with carb rebuild kits.

The rubber tip hardens and wont seal when the bowl fills with fuel.  This causes the fuel to continue to flow and the bowl to overflow.

There is indeed a gasket between the needle seat assembly and carb housing it threads into.If you forget to put it on you ,ll flood out.