Strange. We haven't even met Princess Briet!
I'm guessing that you automatically gain an alliance with them if you reject Astrid and her alliance?
Aranka and the Gauls are diferent people!
Ankara is the one threatning the gauls fallowed by the Danes!
But if you ally with them while marrying the Danish princess making the Gaulish princess your concubine you can get the danes to back off!
But your goal should be Britania and getting the Danes to gift you Britania as your wedding dowry!
it should compensate the loss of Dacia to the horse riders of Valentina mistress of arms
which should greatly increase your personal military power without needing any of the 3 factions!
ofc a part of the Danish princess dowery if we fallow bizantine example should be 6.000 varingians (or 1 legion give or take)!
but if you can choose to give that up for a Gaulish Alliance it might be worth while!
altough the question is.... why Britania must be recovered and not Gaul?
it might be interesting to attract the gauls into conversations betray them and take over Gaul...
this way you get your own personal province but recruiting there might be hard!
but making their remaining princess your concubine and putting an heir in her as always been the norm!
this way you can get the Danish power and more land!