Grolantor is one of the third generation of giant gods, disdainfully called the "runts" by their elders - the other is Karontor. He was banished to Carceri by his father Annam for allying himself with treacherous Memnor, but now that Annam has retreated from the rest of the multiverse, the ban no longer applies. He now splits his time between Carceri and the+Abyss.
Grolantor is evil second and stupid first, disowned by his brothers for his foolishness and relative weakness. Some say he created the race of hill giants by collecting and interbreeding the runts of earlier giant broods, then further polluted this stock by mating with various serpents, medusa-like hags, and the goddess Cegilune. The ettins are said to have descended from Grolantor and a monstrous serpent with a head on both ends of her coiled body. Others name the first hill giant as Ruk, a son of Annam and Othea.
He is worshipped by hill giants, ettins, and ogres. His Realm in Cathrys, the second Layer of Carceri, is called the Steading. The orbs above his realm are ruled by Shator gehreleths, a weak-willed, toadying lot that cede to him their vassalage, more out of fear of the giant Petitioners that make up the armies of Grolantor than out of any love for the hill giant god. His realm itself is little more than a wooden house.
Sources: Complete Divine, Defenders of the Faith, Deities & Demigods (First Edition), Giantcraft, Planes of Conflict,
http://www.wizards.com/rpga/downloads/LG_Deities.zip, Monster Mythology, On Hallowed Ground, Manual of the Planes (1st edition)