Interesting. I think it doubtful that any survived of Fingon's people to flee to Gondolin and the protection of the new High King of the Noldor in Exile (Turgon).
As I understand it, Fingon withdrew from Fingolfin's greater realm to focus on Mithrim and sustain that realm (one of the most ancient Nolorin encampments in Middle-earth). Mithrim did become the vanguard of the Eldarin State in mortal lands. But, after the Nirnaeth Arnoediad, where Fingon was slain, it is said all his people were slain with him.
Mithrim was later occupied by Easterlings and never regained.
Yet, in the telling of the Lay of Turin Turambar, it is said that Rian, wife of Hurin's brother Huor, fled into the wild after no tidings could she get of her husband. This is following the Nirnaeth Arnoediad, and she was aided by the Grey-Elves of Mithrim.
So, by that, not all residents of Mithrim went with Fingon to the Fifth Battle. Some remained and aided the Edain in the immediate years following. It is then Easterlings leaked into the lands of Mithrim.
Still later on, in the opening passages concering Tuor and the Fall of Gondolin, the following is said:
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It has been told that Huor the brother of Hurin was slain in the Battle of Unnumbered Tears: and in the winter of that year Rian his wife bore a child in the wilds of Mithrim, and he was named Tuor, and was taken to foster by Annael of the Grey-Elves, who yet lived in those hills.
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From that, I think that Fingon's host at the Fifth Battle was completely slain. Those left at Mithrim seemed to have faded away. Some took to the hills. I wonder if others attempted to re-settle in Doriath or Gondolin as Easterlings pervaded Mithrim.
Given that Morwen sent her son to Doriath, and she was kinswoman by marriage to Rian, it's possible there was a diplomatic link between the Edain and mayhap Eldar with Doriath. So, in the absence of anything else that I can find, I am leaning towards the survivors that did leave Mithrim drifting to Doriath. After all, they are Grey-Elves and it seems logical that they would seek out their kin.
However, that is all conjecture. I'd be interested to see if a definitive answer could be arrived at.