She never got the chance to tell her mother that she forgives her.
When she tells people where she's from, they immediately assume Gaila hates her mother. Because they can sit there and read about Orion culture all they like, say they're accepting and understanding, but when it comes down to it no human can understand how a mother would sell her own child. So they ask Gaila if she's angry at her, and then when she says she's really not they get this look in their eye like they don't believe her, but they'll humor her anyway.
What Gaila doesn't tell them is that she was angry, and that is the shocking thing.
She was barely eight when her mom shipped her off to the highest bidder, and Gaila hated her. She wasn't like the other kids. She was insanely bright and always scurrying around, fiddling with gizmos and gadgets she shouldn't. In hindsight that was probably why her mom sold her so early, didn't wait until she was ten like the other girls. But the main difference between her and everyone else was that she really didn't want to be there.
When the Starfleet officers were waiting their turn with her mom, when she was too young to service them and instead became an audience for their outlandish stories, she looked at them and knew that was what she wanted to do. She didn't want to be stuck being some slave for whoever paid her the most. She wanted choices. She wanted freedom. Most of all, she wanted the stars. And every Starfleet officer that patted her on the head and winked before stepping into her mother's bedroom passed a little of their passion onto her. The thing that separated her from every other Orion was that she knew she deserved them.
So when her mother ships her off to work in the nearest pleasure palace until she's old enough to be a 'courtesan' (apparently naming themselves after ancient earth prostitutes makes it classier) Gaila is upset, angry, full of hatred. She figured that her mom loved her, cared about her and wanted the best for her. All the moms in the stories she read did. And sometimes her own mother would be like them, would comb her hair and say she was beautiful. Sometimes at night, she seemed almost human.
But as she realized she'd never see her mother again, and she was the only one crying over that fact, she knew the woman who gave birth to her would never be human.
Years down the line, Gaila is okay with that. She's standing in the halls of Starfleet for the very first time when that realization strikes. She aces the entrance exams obviously and they invite her for some initiation thing which basically translates as standing around having the obvious pointed out to them. But Gaila is so unbelievably happy that her dreams are coming true that she doesn't care. She listens, she takes it all in, she even makes notes! The boys around her are shooting smiles left, right and centre because she can't keep a control on her pheromones but it doesn't matter because she's here.
Then a noise from the side interrupts her thoughts. Nobody else seems to be bothered, but Gaila is a curious person so she surreptitiously sneaks a glance to the side to see the cause of the distraction. Standing in the courtyard a few hundred yards away is a Vulcan. She can tell by the point of his ears, the uptight way he's holding himself. With him is another Vulcan, and a human woman. Normally she wouldn't care because Vulcans are the most painfully boring creatures in the entire universe, all blah blah logic blah blah intellect. Gaila can appreciate brains but come on, that appreciation only goes so far on a cold, lonely night.
But then she catches sight of the way the human woman is holding onto the older Vulcan like they're a couple and she thinks, hello this Vulcan isn't quite as uptight as she thought. And the other one looks too like them both for it to be a coincidence. Clearly this is a happy little family, and if there's one thing Gaila loves it's family. Surprising since she never had one.
The guide isn't saying anything particularly interesting, so Gaila sneaks a little further away from the group and closer to the Vulcan Bunch. She doesn't mean to overhear, she just wanted a closer look. But she does, and she's only Orion.
"Sylek has completed the ritual of Kohlinar." the older Vulcan comments, causing the human woman to frown up at him.
"My good wishes to him." the younger one replies, just as even and smooth his father. Wow these Vulcans even speak boring, Gaila thinks, ready to pay attention to the guide. At least he was animated.
"He will do well. I predict that he shall take my place at the High Council, when it is my time to pass."
Gaila assumes that Sylek is this guy's older son because he's babbling on about him like a proud parent whose kid just took their first steps.
"He was always great in intellect, and indeed in physical prowess. I recall his attempts to elicit a physical response from me after classes was most effective." It's delivered in the same smooth tones as before, but even Gaila can pick up on a "fuck you" when she hears one.
The older Vulcan raises one brow, then turns to look out at the rest of the Academy. "I assumed Starfleet Academy would be more... advanced."
"Sarek." The woman had finally spoken up, and Gaila could see her hand tighten around his. She turns to face her son, a warm smile on her face. "We're so proud of you Spock, aren't we?" Sarek says absolutely nothing, simply breaks away from the group to wander over to a display meant for the new Cadets. The woman sighs and in turn picks up Spock's hand, attempting to envelop him in an embrace. He allows her to do so, but makes no move to reciprocate.
"Pride is illogical. I have done nothing to earn such a sentiment. Perhaps disappointment would be a more appropriate suggestion." His voice doesn't waver, but Gaila can sense the disappointment of his own in it, the wretched feeling of knowing a parent doesn't approve.
As she wanders back to the group, she realizes that family doesn't always mean you're loved. She can't escape the fact that she was born different, that her mother had different expectations than she did. It was only natural for her to react the way she had.
So right then and there, Gaila decides that she forgives her mother because there was nothing else for her to do. It was how things were done for thousands and thousands of years. It was only natural.
This is what Gaila tells people when they ask how she can't hate a woman who sold her. That it's just her nature. She made peace with it a while ago, and as soon as she can find her, she's going to tell her mother that she doesn't mind, she's okay with it.
But she never gets the chance to find her.