You can play them in any order you want. Playing Like a Dragon 7 first or before Like a Dragon: Gaiden won't affect your enjoyment of either story. All the Like a Dragon games were designed to be self-contained stories for most part. Play whichever game is tickling your fancy the most.
@@HAZEplusdont listen to the person above. This game spoils major plot points from Y7, play that one first so u can experience the story of y7 blind with all the plot twists in tact