Hmmm, I believe it was Ulmo who said at the time he led Turgon and his followers to the site where Gondolin was built that Gondolin would fall.
Quote:
Then Turgon prepared to depart from Nevrast and leave his halls in Vinyamar beside the sea: and there Ulmo came to hime once again, and spoke with him. And he said: "Now thou shalt go at last to Gondolin, Turgon; and I will maintain my power in the Vale of Sirion, and in all the waters therein, so that none shall mark thy going, nor shall any find there the hidden entrance against thy will. Longest of all the realms of the Eldalie shall Goldolin stand against Melkor. But love not too well the work of thy hands abd the devices of thy heart; and remember that the true hope of the Noldor lieth West and cometh from the Sea."
And Ulmo warned Turgon that he also lay under the Doom of Mandos, which Ulmo had no power to remove. "Thus it may come to pass," he said, "that the curse of the Noldor shall find thee too ere the end, and treason awake within thy walls. Then they shall be in peril of fire. But if this peril draweth nigh indeed, then even from Nevrast one shall come to warn thee, and from him beyond ruin and fire hope shall be born for Elves and Men. Leave therefore in this house arms and a sword, that in years to come he may find the, and thus shalt thou know him, and not be deceived."
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So from that, a few things seem evident. The Curse of the Noldor meant that Turgon would be betrayed from within. Ulmo believed it likely Gondolin would fall, whether Melkor found it himself or the Doom of Mandos worked to that end given enough time. It was for this reason, the arms and helm that Tuor bore to Gondolin later were left at Nevrast.
Based on this, had Hurin never wandered calling and searching near Gondolin, had Maeglin never sucuumbed to evil, Gondolin was still doomed to fall before a single stone had been lain down to form those bright city walls so fair. Such was the Doom of Mandos and so the will of Illuvator.