The gun didn't really make her panic. It made her more on guard, more alert. Batgirl had found her self facing down many gun barrels all of her life. 98% of the time she dodged the bullets no problem. It helped when you could read exactly when and where people were going to shoot simply by reading their body. But with the gun in the Joker's hands, it was more nerve racking, simply because it was him.
"Perfectly." she replied, her eyes not on the gun, but watching him, so she would know when and if he was going to fire. When he surged forward, she was off the bike before he had really gone anywhere. The machine could take care of itself. With so many tools and gadgets inside of it that would work when she wanted them too, anyone being near it wasn't a problem.
If she was going to hit him, it would be with her fists. Batgirl wasn't about to scratch up her bike on a psychopathic clown.
All too well she knew the story of the first Batgirl, she'd asked Barbara to tell her stories of her time wearing the old costume. But Cassandra was nothing like Barbara. The new Batgirl was something entirely different. Wait for him to make the first move, so if he fired, she could react. "Come on, Joker. Going to shoot me down too?"