Only the Danes send one of their daughters (and a alliance offer), Estrid being the oldest daughter.
In King Brennus Kingdom the oldest child will inherit the throne, which is his daughter Etta. All we know is, Etta was send on diplomatic missions before (to other babaric tribes), but he won't marry out is heir, that wouldn't make sense.
The Gauls have Briet which is eligible to marrige. Beside that we know the peace treaty with will expire soon, they have significant fear Caudium will betray them and they have sent no envoys as of now. If they will send their daughter for a marrige alliance under that conditions who knows.
Beside that their Kingdom is small, everyone hates the
french gauls, that being the Danes, king Brennus, Caudium isn't fond of them, neither the MC and as they are responsible what happend to Flora and her family, she would probably try to kill their princess
Who knows, Avila seems not particular liked as even her royal court is considering asking for help from Caudium to get rid of her, her son needs to die thought.