Gannon and Chen will be both be introduced by Felix when the bazaar level reaches 25%. After this, they will appear at the bazaar and each have their own stand.
Chen sells seeds and other various cooking and food items at his shop. The seeds he sells are unique ones that can only be purchased from his shop. The seeds will be random based on the season it is in the game until you reach year 4. After year 4, Chen will sell all of them weekly.
His shop opens at 10:48 and closes when the bazaar is over. Because he can only be seen on bazaar days, he cannot be befriended.