Unlike other stationary giants, Grok won't become hostile when approached but no interaction with him is available. The Dragonborn is forced to kill or run from Grok to retrieve Gleda the Goat. Grok drops standard loot for a giant.
It's also possible for the Dragonborn to sheathe their weapon after retrieving Gleda the Goat and Grok will become passive again, then just take the Goat and leave without any complications. Alternately, he can be beaten into submission and while he is crouched down, take Gleda and get out quickly. His health can also be chipped away with arrows from a distance. A Nord may use Battle Cry to scare Gleda away and then sheath their weapons to make her friendly again, it will not make Grok hostile.
He can be found roaming around the southern and eastern parts of Rorikstead, if spared.