O exército de Morgoth levou prisioneiros em Gondolin?

Bem, eu não usaria a palavra "prisioneiros", mas é muito provável que Morgoth tenha levado alguns elfos vivos como escravos.

Escravos

A tomada de escravos não é incomum para Morgoth; ele faz isso com bastante frequência:

The Orcs growing ever bolder wandered at will far and wide, coming down Sirion in the west and Colon in the east, and they encompassed Doriath; and they harried the lands so that beast and bird fled before them, and silence and desolation spread steadily from the North. Many of the Noldor and the Sindar they took captive and led to Angband, and made them thralls, forcing them to use their skill and their knowledge in the service of Morgoth.

^{_{The Silmarillion III Quenta Silmarillion Chapter 18: "Of the Ruin of Beleriand and the Fall of Filgolfin"}}

E novamente na Batalha das Lágrimas Não Numeradas:

The remnant of the Eldar of Hithlum were taken to the mines of the north and laboured there as thralls, save some that eluded him and escaped into the wilds and the mountains.'

^{_{The Silmarillion III Quenta Silmarillion Chapter 20: "Of the Fifth Battle: Nirnaath Arnoediad"}}

E no saco de Nargothrond:

The Orcs had slain or driven off all that remained in arms, and were even then ransacking the great halls and chambers, plundering and destroying; but those of the women and maidens that were not burned or slain they had herded on the terraces before the doors, as slaves to be taken into Morgoth's thraldom.

^{_{The Silmarillion III Quenta Silmarillion Chapter 21: "Of Túrin Turambar"}}

Então, embora ele estivesse muito chateado com Gondolin por tê-lo iludido todos esses anos, não seria improvável que ele tivesse levado alguns escravos para lá também.

Gondolin

Infelizmente não há nada no Silmarillion publicado para indicar que Morgoth fez escravos em Gondolin. Então temos que voltar aos rascunhos, e temos que ir longe de volta.

O principal relato da Queda de Gondolin é dado em O Livro dos Contos Perdidos , e foi escrito em 1917. Como tal, tem muitas inconsistências com o resto do Legendarium, e se ou você não considera que canon é uma decisão pessoal. No entanto, ele contém uma indicação de que os escravos foram levados pelas forças de Morgoth:

Thus it was that Tuor son of Peleg gave before the foe, fighting as he yielded ground, and bore from that battle Ecthelion of the Fountain, but the drakes and the foemen held half the city and all the north of it. Thence marauding bands fared about the streets and did much ransacking, or slew in the dark men and women and children, and many, if occasion let, they bound and led back and flung into the iron chambers amid the dragons of iron, that they might drag them afterward to be thralls of Morgoth.

^{_{History of Middle-earth II The Book of Lost Tales, Part 2 Chapter III: "The Fall of Gondolin"}}

Claro que este não é o fim da batalha; logo aprendemos que Eglamoth tem resgatado alguns cativos:

Even from that region comes now Eglamoth. He had had a charge of the engines on the wall; but long since deeming matters to call rather for handstrokes about the streets than shooting upon the battlements he gathered some of the [House of the] Arch and of the [House of the] Swallow about him, and cast away his bow. Then did he fare about the city dealing good blows whenever they fell in with bands of the enemy. Thereby he rescued many bands of captives and gathered no few wandering driven men, and so got to the King's Square with hard fighting.

^{_{History of Middle-earth II The Book of Lost Tales, Part 2 Chapter III: "The Fall of Gondolin"}}

Então, é certamente possível que Morgoth não tenha conseguido tirar cativos de Gondolin, mas parece que ele estava pelo menos tentando.