Gothmog was the son of Melkor/Morgoth Only in the very early drafts. In later versions, including the published Simlarillion, there is no familial relation.
Captain of Angband (Sauron was only Morgoth's wimpy lieutenant)
You're comparing the words "lieutenant" and "captain" as if they're an O-2 and an O-3. They're not. Gothmog was merely a fighter charged with keeping the Balrgos in line, Sauron was consistently left in command (of Angband and Tol-in-Gaurhoth) when Morgoth wasn't around.
and he did have a fell hammer. Killed some snotty elf-lord with it.
Gothmog wielded an axe, though he probably also used other weapons, such as the whip and sword. Grond, the hammer, was Morgoth's weapon.
In Return of the King, Sauron's weak minions named the giant ram they used to knock down the gates of Minas Tirith "Gothmog."
They named it Grond. Gothmog was the person who took command of Sauron's troops when Angmar was killed.
At the risk of sending this into a total geek-fest I will say, I was wrong about the hammer and ram (it's been a long time), but sorryman was never my boss. It seems there is a lot of mystery surrounding Gothmog. From a 2 minute search I get this:
http://valarguild.org/varda/Tolkien/encyc/maiar.html#GothmogBalrog
Gothmog
by Anarion -(V)
Jan. 17, 2000
lit. - The Voice of Goth (ie. the voice of Morgoth).
Gothmog, originally conceived as the son of Morgoth, although Tolkien later changed his mind, was the High Captain of Angband and Morgoth's most supreme lieutenant. Morgoth caused the downfall of many great people, slaying Fëanor, and Ecthelion. Although Gothmog died in this act too, as they both drowned in the waters of the Fountain of the King.
Gothmog was the most powerful of the fallen Maiar spirits known as Balrogs.
References - The Silmarillion, The Book of Lost Tales II
Gothmog
by Eru-(Valar)
Oct. 26, 2000
Gothmog ("Hateful Enemy") - the chief of the Balrogs and Warlord of Angband was a hideous manifestation of ire and darkness, forever tied to the terrible legacy of Melkor. Third in the hierarchy of Darkness and possessing a malicious intellect and cruelty above all but Melkor and Sauron, he was the flaming fist with which Melkor struck at his Elven enemies. Unfortunately for Gothmog, he met his match in Echthelion and died even as he slew the mighty Elven lord in the sacking of Gondolin.
References: mostly Silmarillion
Fallen Maiar
Gothmog
by Varda-(Valar)
Dec. 19, 2002
Two characters named Gothmog are mentioned in Tolkien's writings. One is the Balrog of the Silmarillion, kept from the earlier version found in Book of Lost Tales. The other is probably named for the Balrog and is the Lieutenant of Minas Morgul in the Third Age, spoken of in the Return of the King.
In BoLT, Gothmog was the son of Melko (Melkor/Morgoth). In later versions, none of the Valar had children, so this concept was dropped. But in BoLT, this gave him special status as all of those people opposed to the Valar of Valinor were under Melkor. His mother was called Ulbandi, in one spot, and the ogress Fluithin in another. Gothmog was also called Kosomot. As the book goes on, we find that Kosomot is Gothmog, Lord of the Balrogs. In BoLT's name list for the Fall of Gondolin, Gothmog means "Strife-and-hatred" with mog as "detest/hate" in the Gnomish (Noldorin) dictionary, and he's called "Captain of Balrogs". A later name list in the 1930's shows goth as meaning "Lord" or "Master". In Eldarissa (Eldar language distinct from Noldorissa), Gothmog is known as Kosmoko and Kosomok(o)
In BoLT2, he is prominent in "The Fall of Gondolin". It speaks of him as "lord of balrogs, captain of the hosts of Melko". In another, BoLT1's "The Coming of the Valar", he is called a "marshall " of the hosts of Melko.
At the Fall of Gondolin, Gothmog had the metal dragons (robots) of Morgoth pile themselves at the northern gate of Gondolin and knock it down. The top metal beasts "opened about their middles, and an innumerable host of the Orcs, the goblins of hatred, poured therefrom into the breach...". (In later stories, dragons are organic, not metal.) Later, when the elf, Rog, and his people called "The Hammer of Wrath", attacked and killed balrogs, Gothmog stepped in again. He had a group of his demons attack from the front, then fall back to lure them between two much larger flanking forces. The elves were supposed to try to pull back, but instead attacked more fiercely forcing onward and destroying those who lured them, then killed many bands of the besiegers, until they were at last overpowered by numbers of Orcs and Balrogs and by a firedrake. The loss of Rog's force hurt the morale of the elves, gave Melko's forces the gate, much of the walls, and good part down to the center of the city. A firedrake and Gothmog together beat down Tuor who had saved Ecthelion, so Ecthelion rose up and attacked Gothmog, driving the spike of his helmet into Gothmog's chest and toppled him over into the king's fountain so that they fell together and their weight took them deep and drowned them.
The "Fall of Gondolin" story changed much between the time of the BoLT and the Silmarillion.
As the stories developed, Gothmog, no longer the son of Melkor, became the slayer of Feanor. Then in the Battle of Unnumbered Tears it was Gothmog who slew Fingon and captured Hurin.
In the Return of the the King, in the Third Age of the Sun, Gothmog was the name of the lieutenant of Minas Morgul.
References: BoLT, Silmarillion, Return of the King
Since I was there, I'll tell you this part is right: Gothmog was "Morgoth's most supreme lieutenant."
Sauron named his mouthpiece after me just as a jab, but he ain't around anymore, is he. Ha ha ha.
Also from here:
http://tolkien.cro.net/balrogs/names.html
Did Any of the Balrogs have Names?
From: Erik Tracy
Yes. We do know the name of at least one: Gothmog, Captain of Angband, Lord of Balrogs. It was he who slew Fëanor and Fingon. And during the Fall of Gondolin, Gothmog slew Ecthelion but was himself slain.
The following quotes are from the Silmarillion:
"Thus it was that he (Fëanor) drew far ahead of the van of his host; and seeing this the servants of Morgoth turned to bay, and there issued from Angband Balrogs to aid them. There upon the confines of Dor Daedeloth, the land of Morgoth, Fëanor was surrounded, with few friends about him. Long he fought on, and undismayed, though he was wrapped in fire and wounded with many wounds; but at the last he was smitten to the ground by Gothmog, Lord of Balrogs, whom Ecthelion after slew in Gondolin."
"Gothmog, Lord of Balrogs, high-captain of Angband, was come; and he drove a dark wedge between the Elvenhosts, surrounding King Fingon, and thrusting Turgon and Hurin aside towards the Fen of Serech. Then he turned upon Fingon. That was a grim meeting. At last Fingon stood alone with his guard dead about him; and he fought with Gothmog, until another Balrog came behind and cast a thong of fire about him. Then Gothmog hewed him with his black axe, and a white flame sprang up from the helm of Fingon as it was cloven. Thus fell the High King of the Noldor; and they beat him into the dust ywith their maces, and his banner, blue and silver, they trod into the mire of his blood."
I Rule! Ha ha ha.