It’s the first non-VIP gem challenge event since the last update.
I have a lot of gold but I value gems more. So my strategy is to collect as close to the 650 gem limit without going past that limit unless I roll a 20. If you time it well you can get almost 900 gems. There is a chance you roll 3 gem dice and it will force you past as well, but that chance has decreased since the update because gem rewards represent less than half the rewards now.