You can take a shortcut and skip to the next milestone by paying Gems. The cost in Gems is determined by the number of Trophies needed to fill up the counter. The more Trophies are missing the more Gems you have to pay. Press the 'Complete' button and then confirm your choice. This will instantly fill up your Trophy bar and grant you the next milestone rewards.
 
This option is available as long as the Tournament is running.

This option is available for every player, purchasing the Premium Nascar Pass isn’t required.