Ok, this could all be nothing or have a story behind it that I am not aware of, but is Gao's name misspelled in the sign of HIS shop?
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At first I thought that Guo might be Cain's father and Gao's son, but considering that Gao is the one who makes all the potions in the shop and that he calls the store HIS life's work, I doubt that that's the case. And if Guo was Gao's father, then I am surprised that they had modern signs that many decades ago or that they kept the name despite the changes in generations.
I think they misspelled Gao's name, though I have no proof, but if they really did, then I feel really sorry for Grampa here.
His grandson scares away any customer that enters the shop, the people perceive him as a scammer because a product as good as his cannot be real, and they even have his name misspelled in the sign of his store. Luckily for him, he just hired the right underage 40-something-year-old teenager to fix it all

. (I never thought that sentence to be possible, but I guess that's the magic of this game).