Maybe. Check whether your existing bed protection actually covers the inside face of the tailgate. Some bed liners and mat systems include tailgate protection, while others stop at the bed floor and sell the tailgate piece separately. WeatherTech, for example, offers an additional tailgate piece with its TechLiner system.
If your tailgate is already protected, you probably don't need another liner. If it remains exposed and you regularly slide tools, lumber, equipment, coolers, bikes, or other cargo across it, a separate Tailgate Liner can protect that loading surface without replacing the rest of your bed setup.