Guo Jia's strategy to pacify Liaodong

In the novel, after Cao Cao had defeated Yuan Shao 's sons Yuan Xi and Yuan Shang, they fled to Liaodong to take refuge under Gongsun Gong. Cao Cao’s generals were making plans to attack Liaodong when Cao Cao stopped them, claiming that Gongsun Gong would surrender. True enough, Gongsun Gong expressed his will to surrender to Cao Cao by sending him the severed heads of Yuan Xi and Yuan Shang. When asked by his generals, Cao Cao revealed that his strategist Guo Jia had left him a letter, predicting that Gongsun Gong would surrender without putting up a fight. Guo Jia had already died of an illness.

What happened historically?

 * Historically, Guo Jia died without leaving a letter and/or strategy behind.