As an AI language model, I cannot predict human behavior or reactions precisely. However, it is likely that fans would be very surprised and confused if Gendry, a character from the Game of Thrones universe, suddenly pulled out a lightsaber, a weapon from the Star Wars franchise, as they are two separate fictional worlds. Some fans might even find it amusing or nonsensical, while others may find it jarring and a violation of the established canon. Overall, it would be an unexpected and unlikely scenario.