Gellick was sent to Dread Mountain long before the Belt of Deltora was broken; the Dread Gnomes stated that the Emerald appeared on Gellick's brow "only" sixteen years ago, indicating that he had ruled over them for much longer than that.
Lief, Barda, Jasmine first observed Gellick with the help of the water from the Dreaming Spring. In their dreams, they watched the Dread Gnomes feeding the immense toad countless flies as Gellick continually demanded more. When an old Dread Gnome fearfully told Gellick that they would need more time, Gellick spat its venom at the gnome, killing him. The toad demanded that the prisoners – Lief, Barda, Jasmine, and Prin – die. It threatened the Dread Gnomes and sent them to collect more flies before falling to sleep, as Lief finally noticed that the Emerald was set in Gellick's brow.
Lief, Jasmine, Barda, and Prin were able to escape from the Dread Gnomes' trap and managed to convince Gla-Thon and Fa-Glin to help them kill Gellick by reminding them of their tribe's former glory. The group, along with Ri-Nan, snuck into the treasure room where Gellick slept; they planned to throw Blisters they had collected at the toad. Ri-Nan betrayed them, waking Gellick, and the Blisters were ineffective, as they were made from Gellick's poison. Ri-Nan attempted to convince Gellick to kill Fa-Glin and make him leader of the Dread Gnomes.
Gellick, insulted by Ri-Nan commanding it, focused its attention on the gnome and killed him with its venom. However, Lief and the others had used the distraction to hide under the treasure in Gellick's room. Gellick boasted about its power before lumbering through the treasure, intending to crush the intruders. Just as Gellick uncovered Lief's hiding spot, Lief threw his bottle of Dreaming Spring water into the toad’s mouth. The enchanted water transformed the evil toad into a massive tree like the ones surrounding the Dreaming Spring, and the Emerald fell from the tree into Lief's hands.
Gellick's death effectively ended the Grey Guards' supply of Blisters, though the Dread Gnomes fooled the Guards for a while by giving them stream water mixed with sap of boolong trees.
Gellick was a gigantic Ooze Toad with lumpy, oozing flesh, slimy, clawed feet, and a long, red tongue. Its skin constantly oozed poison, which the Dread Gnomes collected. The Emerald was set in the middle of its brow. When Gellick swallowed the water from the Dreaming Spring, it transformed into a massive, pale tree with three branches.
In the anime, Gellick is slightly different in appearance. His body is bluish gray in color, he lacks the horn-like protrusions above his yellowish, more amphibian-looking eyes, and he retains his clawed feet. He also seems to sport a pair of fleshy growths on both sides of his mouth.
Gellick was a horrifically greedy, gluttonous, and cruel creature. It forced the Dread Gnomes to feed it with an unfathomable number of flies, only to brutally kill them if they were unable to keep up with its ravenous appetite. Jasmine compared Gellick to the Wennbar, but commented that Gellick killed for spite, rather than for food. It normally spoke by bellowing commands at the top of its lungs, only rarely lowering its voice. It took great pleasure in its power over the Dread Gnomes, tormenting them with threats. Gellick claimed that even the Shadow Lord respected it, a level of pride that few of the Shadow Lord's servants dared to demonstrate.
Gellick's massive size offered it great strength and some protection. Its hide was too thick for swords, axes, or arrows to pierce in. Its poison was its greatest weapon; by spitting at its targets, it could cause them to die in agony very quickly. Gellick is immune to its own poison. The venom retained its toxicity over time, allowing the Dread Gnomes to use it on the tips of their arrows. Gellick's poison can be cured with the Ruby from the Belt of Deltora.
Gellick is the only known victim of the Dreaming Spring's water that was transformed away from the Dreaming Spring.