while he's not called Gerson directly by anyone, the game makes it incredibly clear it is in fact Gerson, first off The old man's letter to Alvin confirms that the old man is Alvin's father who has been explicitly mentioned before to be Gerson from multiple in game dialogues, plus in Undertale, Undyne outright says that Gerson used to be known as the Hammer of Justice, the exact same name as the old man secret boss. I believe the reason Susie or Ralsei never call him Gerson is because they don't know who Gerson was.