gaius ships it

Gwen had surely remained upright and strong throughout, having lost all those who had once held her, she would have been alone in her grief. Perhaps Gaius had held her close. Merlin hoped he had.

Now it was his turn to hold together someone else, to keep her from shattering apart and dissolving into nothing. Gwen clung to him and together they rocked, swaying with the sorrow of being the two people in all of the world who knew what it was to be loved by Arthur. A gift incandescent in its giving, unbearable in its loss.

When Tharja met Robin, she immediately set to work placing a hex that would let her and her beloved be reincarnated after death, so if things didn't work out, she'd get another chance. By the time she ended up falling in love with and marrying Gaius, she'd forgotten all about the hex, but the effect still happened. She and Gaius were reincarnated as Asugi and Rhajat and were together again in that life.