After much soul-searching and review of canon sources, I decided to go with Fingon as Gil-Galad’s daddy.
Both Maedhros and Fingon found it strangely satisfying that the heir to the line of the High Kings of the Noldor, the latest issue of the children and grandchildren of Finwë, was half Sindarin.
“Such a pretty baby,” Maedhros said, drawing the infant closer to his chest with a skill gained in handling several younger brothers and cousins in another world and another Age. He lifted his incomparably handsome head to smile with great tenderness at Fingon, allowing a red braid to fall within easy reach of a plump infant fist. “He looks uncannily like you did on the day of your Essecarmë.”
“You recall how I looked then?” Fingon asked, the open-mouthed grin and glint in his eyes clearly showing that he was pleased with the idea that it might be true. “His eyes are silver, not blue,” he added.
“So were yours when you were born, as were Tyelcormo’s. One cannot tell what his eye color will be yet. They could still turn blue like yours or silver-grey like his mother’s.”
“Or, for all we know, he could look like you.” Fingon smirked.
“I doubt it. He looks exactly like you did on the day of your Essecarmë.”
“I suppose it is well that he does not have red hair.” They laughed, for the moment carefree as the youth they had been under the golden light of Laurelin.
“Are you happy, Káno?”
“I miss you constantly. I think of you dozens of times a day. But I am happier than I expected I could be away from you, although I always count the days until I see you again.”
“I am here now. I promise I will stay longer this time. Now take me to your lady wife so I may congratulate her on how beautiful is her son.”