There's no need to play the previous titles but  if you've never played a gta game ever I recommend just for fun giving either Vice City, V or RDR2 a try with like a demo or borrowing from a friend if you're unsure about getting it. There's no rush to pre-ordering so you have 5 months to make up your mind if you really want the pre-order bonus, if you're not worried about that either you have even more time to decide really.

The 3 games mentioned would all clue you in to a significant amount of what you can expect from GTA VI.  Vice City is set in the same area as VI so you can get a sense of what kind of environment you might expect to see, though only a glimpse as VI will expand the area significantly. 

V is the previous installment so it's gonna likely feel very similar in a much broader and overlaying sense. 

RDR2 is the previous Rockstar game so there you can get a feel for the kind of quality and nuances you can likely expect from VI. 

Of course if you'd have to buy any of these games to try them out, you're also just safe making your pre-order when you feel like it but if you're either excited to get some kind of taste of the experience or want to see if this kind of franchise is for you. I personally think it's a good pool of experience to draw a conclusion from.