What if the Fall of Gondolin was won by the Elves?
It would have just delayed the inevitable because the Curse of Mandos and the Oath of Feanor were just too powerful.
Even if Turgon and/or the Gondolindrim managed to beat Morgoth's assault forces, it would still have been doomed. The greatest strength of Gondolin was it being hidden. So long as Morgoth didn't know where it was, she was safe. She could thrive. Her biggest defense wasn't her armies nor even the Great Eagles of Manwe that lived above the city: it was the Encircling Mountains.
Whether under Turgon (if he survived the battle) or any other leader (probably Tuor), the Gondolindrim would have had to abandon their magnificent city.
After that, where else is safe?
At the destruction of the Noldorin Realms in Exile, the Falas fell next. Menegroth and Doriath would follow fast due to the Curse of Mandos driving it.
And perhaps the only reason why Morgoth didn't deign to finish the last of the Noldor and Sindar at the Havens of the Mouths of Sirion was because he couldn't be bothered. And the Sons of Feanor did the work for him, anyway, in their vain quest to wrest the Silmaril recovered by Beren and Luthien from the hands of their and Turgon's heirs.
Gondolin was not meant to stand, after all. Turgon was told this by none other than Ulmo, Lord of Waters, himself when he was guided to the Vale of Tumladen. The strength and valor of the Noldor will not suffice because the Curse and the Oath were too powerful. Gondolin was meant to stand for a little while to, as Huor told Turgon on the lost battlefields of the Nirnaeth Arnoediad, give birth to the hope of the Eldar and Edain.
Ulmo even expected his chosen Noldorin king to forget, which is why the coming of Tuor was foretold. He was the sign to Turgon that the Doom of Mandos speeds swiftly and the Noldor must seek to repair their relationship with the Valar.
But the newly-crowned High King of the Noldor in Exile had grown fond of his city, said to rival Tirion upon the Hill of Tuna in Eldamar. He tarried. He trusted in the strength and valor of the Noldor.
Even if the Gondolindrim prevailed, their cause was lost. The Curse of Mandos and the Oath of Feanor ensured this. All the works of the Eldar and Edain that followed them would be for naught; they could win for a time, a season, even for four centuries, but the end is the same defeat at the hands of Morgoth.
No: the hope of the Children of Illuvatar had already been born, both in the Hidden City and fallen Menegroth, where Elwing saved the Silmaril her grandparents took from Morgoth's Iron Crown.
Turgon should have fled to the Havens, or established a mighty fortress at the Mouths of Sirion at Tuor's coming, to give his grandson time to bring the Silmaril to Valinor.
By the time Maeglin betrayed Gondolin to Morgoth, the fate of the city, its people, and its king, was already sealed.