It's a theory, whether it's true or not for you depends whether you subscribe to the interpretation or not. From what I have read, most of the reasoning behind this stems from how Gale describes the development of their relationship from teacher to muse to lover, how very young he was when his talents became noticeable, and how firmly his self-worth is tied to his magical skill and general usefulness. 
While I don't agree, I can see where the grooming theory comes from. I think it misses the point though, because Gale's story isn't about exploitation of the body (that's Astarion's side) but about him being reduced and, in turn, reducing himself to his skills as a scholar and wizard. I am glad when he finally gets angry at Mystra, since her orders were - if you value mortal lives - cold on a personal level and monstrous on the larger scale. Obeying her in Act 2 probably destroys more lives than Cazador's ritual - granted, without minting any soul coins.
Where you stand on the matter of guilt also seems to largely depend on whether you have a more devoutly religious or a humanist mindset. The religious crowd seem to be of the opinion that Gale should have listened to the boundaries his goddess set for him, while the more humanist minded point out that he is a wizard, not a cleric, and he was encouraged to develop his magical prowess. Mystra showed Gale what he could achieve while at the same time denying it to him, feeding into his issues. The topic of being "worthy" comes up time and again. Gale thinks he has to proof himself worthy by demonstrating great skill and knowledge, while Mystra & all think he already was worthy because of his good nature. If you wish to refrain from a more nefarious interpretation, this is a serious communication issue at least, one that also rests on the shoulders of Elminster and Tara. 
I like the relationship with Mystra in Gale's Origin much better because it allows for healing and understanding, which is not possible in the scripted audience. I loved that I could call her out for intending to sacrifice so many lives with her orders (which felt like a very Gale thing to do) but also that it provides insight into why she asked for the sacrifice in the first place, making the deal she eventually offers seem like a compromise between her duties as a goddess and her sympathies for Gale. In my game they parted ways as friends and Mystra seemed happy to see him leave and lead his own life without being so closely tied to her influence, while Gale is apparently hunting down rogue shadow-mages again even though he's not a Chosen any longer. 
In the companion version the relationship is probably also less cordial, to keep it level with the stories of the other companions and their gods and masters.
Last edited by Anska; 12/02/24 10:03 AM.