Presumably the game will be announcing imminently if the t-shirts are already at GameStops.
Source: https://www.reddit.com/r/assassinscreed/comments/6e8dm0/leaked_image_from_a_gamestop_shirt/
Source: https://www.reddit.com/r/assassinscreed/comments/6e8dm0/leaked_image_from_a_gamestop_shirt/