How is the gameplay like - like what would you compare this to in terms of gameplay?
(Never played a Yakuza game)
Is it like Sleeping Dogs/GTA?
Not at all, the closest thing to it is probably Shenmue. You cant steal cars and just shoot random people in the streets. You still have a city to explore with sidequests, minigames and all that but really dont go in expecting it to be as open as a GTA. You pretty much walk/run except for special cases and i dont even know if those are in Zero. Like in 5 you were able to work as a taxi driver.
Kind of like in a RPG, you'll have encounters with punks/yakuza whatever, but can run away if you dont wanna fight. If they get to you, you get into "Fight Mode". You can use objects around you, like trashcans and all that, have 3 attack buttons, a grab and a dodge. You can also equip weapons and choose 'em during battle. Unfortunately i cant go into too much detail because i am not sure what Zero does different, so i could tell you things that dont even happen in this game but are in the older ones. Sorry!
Those games are also really story heavy.
Also would it be a good idea to start at 0 before the first game?
I would say yes since it's a prequel and takes place before Yakuza 1 BUT i heard some people say that it is not a good place to start.
Actually, if you have a Ps4 and a Japanese Account, go and check if the Zero demo is still there. You'll get a feel for the gameplay that way.
I'm not good at explaining stuff, so maybe someone will do a better job and tell you what Yakuza is. And if you still have questions ill try to answer as good as i can.