Gift of the Elements allows you to upgrade three main stats of a hero. To level up a hero's Gift of the Elements, you need gold and Sparks of Power that you can get for upgrading titans, their skins, and artifacts, or buy in Titan Soul Shop.
The maximum Gift of the Elements bonus is 720 to the hero's primary stat and 360 to the other two stats.
You can always reset your Gift of the Elements level. Note that you get back only Sparks of Power, not gold.