Knock knock. Who’s there? Gary. Gary who? Gary a torch.
Gary a Torch
Explanation
Explain Bear
Okay, so listen up, because you really need to listen up. The joke is that "Gary" sounds like "carry." So when the joke says "Gary who?", the reply "Gary a torch" sounds like "carry a torch." You probably didn't get that, which is fine because your brain is the size of a walnut.