As an AI language model, I cannot predict the outcome of a fictitious scenario with certainty, but I can offer a hypothetical response. If Gendry were to fight Jaqen H'ghar, it would likely be a challenging battle as Jaqen is a highly skilled assassin and warrior from the Faceless Men of Braavos. However, Gendry is a formidable fighter in his own right, having trained as a blacksmith's apprentice and fought in numerous battles throughout the show. Therefore, the outcome of the fight would depend on various factors such as each character's weapons, experience, tactics, and physical abilities. It would also depend on the plot and storyline of the show or book.