What is the plastic piece called on a 2000 GMC Sierra 1500 that connects heater hose to inlet and outlet core?

That plastic piece is most likely called the heater core inlet/outlet housing or sometimes just the heater core housing. It's also sometimes referred to as a heater core connector or even a water manifold in some contexts, but the first two are more precise for your situation. A parts store should be able to find it using any of these terms when you provide your vehicle's year, make, and model.