Gierrod was a Jotunn. He accidentally crushed himself to death in a drunken fit of rage.
Gierrod comes home drunk to find Magnus Chase, Samirah al-Abbas, Hearthstone and Blitzen in his palace. They ask him if he has Thor's weapon, which they assume to be Mjølnir. He agrees to barter with them for the weapon; if they play a game of catch and survive for ten minutes, he'll let them take Thor's Staff and Gunilla, who had been captured by Geirrod's daughters, Greip and Gjalp. Halfway through Geirrod's very fatal game of catch with hot coals, Magnus tells him that they killed his daughters, which angers him and causes him to throw the coals at the support beams until the ceiling collapses, crushing him to death.