Each Event offers two sets of rewards (or reward tracks) - the top one is the basic reward track that you will be receiving for completing the Event's milestones.
But what if you'd like to boost your Event rewards? In such a case, you can unlock the extra reward track by purchasing the Event Ticket using gems. 🎫
Simply tap the "Get now!" button on the Event pop-up to do so. You can purchase the Event Ticket at any point during the Event (including when you've completed it!) and claim all the previously unlocked reward milestones.