Giants Chests can be earned both as a reward from the Path of Giants and by continuing to collect Giants Points even after all Milestones have been reached.
There is a bar under the "Rewards" tab in the “Path of Giants” menu. Once the player has reached the 30th Milestone, the bar will begin tracking additional Giants Points earned. For every 1000 additional Giants Points, the player will earn a Giants Chest. At the end of each Path of Giants, all collected Chests are automatically claimed and Giants Points are reset to 0.