He served in Sauron's army during the War of the Ring. He was the lieutenant of Minas Morgul, second-in-command to the Witch-king of Angmar, lord of the nine Nazgûl (Ringwraiths). He commanded the forces of Morgul during the Battle of the Pelennor Fields after the Witch-king was slain by Éowyn and Merry.
This is the only reference to Gothmog in The Lord of the Rings, and he appears in precisely one sentence. His fate is not stated, although it is strongly implied that he and almost all of the servants of Sauron that fought before the gates of Minas Tirith were killed. Tolkien writes nothing else about Gothmog — not even what race of beings he belonged to.
Gothmog shares his name with one of Tolkien's earlier characters, Gothmog Lord of Balrogs, who is featured in The Silmarillion.
In Peter Jackson's The Lord of the Rings: The Return of the King film, Gothmog is portrayed by Lawrence Makoare (who also portrays the Witch-king of Angmar and Lurtz) and voiced by Craig Parker (who also portrays Haldir).
Gothmog is interpreted as a deformed Orc with a stunted arm. According to Peter Jackson, Gothmοg's affliction is an elephantiasis-type disease; he dubbed him "the Elephant Man Orc". Jackson asked the Weta Workshop designers to make Gothmog look diseased. Knowing his penchant for outrageous creatures, they piled elephantiasis growths onto a model in such a way that they hoped even he would find it excessive. However, to their surprise, Jackson deemed it to be 'just about right', thus giving Gothmog his "Elephant Man"-type appearance.
In a sequence cut from the theatrical cut but included in the extended DVD cut, Gothmog fights an even duel against Éowyn, but is temporarily incapacitated. While Éowyn lies injured at the hands of the (now dead) Witch-king, Gothmog goes to finish her off but he is killed by Aragorn and Gimli, unaware they have just saved Éowyn.
He is also a playable Mordor hero in Electronic Arts' real-time strategy game The Lord of the Rings: The Battle for Middle-earth II - The Rise of the Witch-king, based on Jackson's films as well as Tolkien's writings; Electronic Arts' "The Lord of the Rings: Tactics"; and in the Evil Mode of Electronic Arts' The Lord of the Rings: The Third Age.