ooooh, this is my favorite question!
Gale is a Circle mage, first and foremost, and being a lot more powerful than most mages, he understands the dangers of uncontrolled magic. That is why he will be dealing with the mages in the Hinterlands a bit more harshly, since they are letting civilians get caught in the crossfire. And since the templars are attacking innocent people, Gale is gonna be hella furious at them too. He would make sure to have both Cullen and Vivienne involved, to help deal with their respective sides.
In general, Gale is pro-Circle, believing in the necessity of having the Circle's structure, but hoping to drastically fix the power imbalances between Mages and Templars.
As for Cullen, Gale, being one hell of a flirt, would try to get into Cullen's pants. But he wouldn't trust a Kirkwall Templar by a long shot. Gale, being from a Marcher Circle most certainly knows how the Gallows went to hell.
I think all of my mage Inquisitors would understand the importance of having some sort of structure and safeguards in place when it comes to magic. But what that amounts to would vary among each of them. I think my circle mage Artair would prefer something more closely resembling the circle and is probably the most pro-circle, especially since that is what he grew up with and what makes him comfortable (except, you know, with less abuse and better people in positions of power). My Qunari mage Veska might agree with him, with the exception that she thinks that more exposure to the outside world would be necessary. She thinks it does no good to stay inside a tower or a fortress and cower. The world won't stop fearing mages if all they do is hide (in her eyes).
(Although I think all of them would want to get rid of the policy that families have to be separated because a parent or a child is a mage.That is one thing they agree on.)
My canon Inquisitor [Magdalene] would think the idea of a school or place where mages could learn to harness their talents would be useful. And as someone who was in training to be the first of a clan, she understand the importance of having a repository of information available. I think she'd want the most freedom of any of my three mages, at least at first. She initially afraid of Templars because of her upbringing but learns that the idea may not be a bad one. But I think she'd come to reconcile and take a more balanced view. Though, of all three, she is the most compassionate and wants to find a solution where everyone is happy and working together rather apart and at odds.
As for their initial reactions to Cullen, I think it would go like this (assuming they think he's a Templar or associate him with such):
Artair: Exasperation. (Oh great, another Templar! Unless you are hear to help, I don't have time for your shenanigans!)
Veska: Caution, but confidence. (I do not know what to expect, but whatever it is, I can deal with it.)
Magdalene (canon): Fearful on the inside but civil on the outside. (Easy now. Don't want to get myself into more trouble. Just stay cool, and maybe we can end this peacefully. I hope...)
Maggy is actually the only one romancing him, I figure she gets over being afraid of him eventually as the two develop a trust in one another and become friends, and then they start to notice each other and develop a closer relationship.