I'll take articulate as a compliment... not sure about formidable though... regardless, I hope no-one ever feels like they shouldn't disagree with me, or that I'm trying to intimidate anyone from doing so... eek!
Gale's delivery is manipulative. [...] He's using the foot in the door strategy
I don't agree with that interpretation of what he does and says - I feel that that is a very prejudiced read of his actions from a perspective that has already decided - in advance - that they didn't like the kind of person that the speaker has already decided that he is.
If I recall, he doesn't say that he can trust you, or that he knows he can trust you - he says that he feels like he can, and as with his personality type (which I'm well acquainted with and share many elements of), expresses that with backed-up reasoning; the merits and actions that he's personally put value on. Yes, he attempts to extract a promise first, and that's not 'nice' - not at all! - but nice and good aren't synonyms, and it
is pragmatic for a very serious issue, when he has noted that you are a person who treats keeping promise as important.
The act of trust
is his telling you, and trusting that you will not act poorly with that information; if he has misjudged the situation, and you will not keep his confidence and refrain from freaking out, that could go pretty badly for him. The promise is a security blanket - for him - because it means nothing, has no value anyway and will
do nothing if he's wrong about whether to trust you with his dilemma or not anyway.
Would it be better if your refusal to promise could lead to dialogue that could in turn resolve with him caving in and confiding in you anyway? Yes, absolutely! But given the issue at hand, and his academic pragmatism, it's not a sign of disingenuous behaviour or intention. Foot-in-the-door is defined by escalation, and Gale does not escalate. What he has to tell you is legitimate, and he lays out the cards of what he needs up front once he does reveal it - he doesn't ask you for anything tangible
Before telling you the details either - he spells it out first, and tells you what he will need, as soon as you're able to provide it. This need doesn't change from the initial description and is not escalated at any point. It is exactly as he describes it to you, and remains that way.
He doesn't start by saying that he just needs one, and then later shift into just one more, and then just that big one... Up front he says he needs to do this regularly, but the stronger the better, and if you DO give him a strong item, he tells you that he believes that should keep him stable until you reach BG - and lo and behold, he never asks for or has any opportunity to offer or accept another one from that point on.
I feel that Gale is not, in fact, 'like' the kind of guy you hate - you're just choosing to see those traits where they aren't really present.
Or perhaps I'm not seeing those traits as clearly because his personality type is one that I am drawn to and appreciative of. I'm willing to entertain either as being a possibility, but, biased as it may be, I feel my footing is more backed up than yours.
Note that one of the few ways you can rid your party of the Gale virus once you catch it is to use the tadpole. If you use the tadpole and get caught and/or admit it he leaves the party. He'd rather leave than be found out.
This might be a hot take, but... If I had an associate who tried to read my thoughts at one point, and I strongly and clearly told them that that was not okay, and not to do that... and they later, when I showed them some trust, violated that request of mine and accessed my inner thoughts and memories against my will... I 100% would absolutely distance myself from them and want no further contact with that individual; if they will not leave, then I will. No thanks, deal broken, I'm out. Do I hold it against any other character for reacting in the same way? No I don't.
I'll also point out that none of the thoughts and memories we can pry into and gain extra gleanings of in any way contradict or run against what he openly tells us - if anything they confirm that he's being honest about what he did and why, and that it's a point of on-going shame and regret for him... in contrast to the way our insights show things of others that they actively try to deny (*cough* Wyll).
The tools I'm referring to here are the interface and structure by which the game lets us determine if a character is being deceptive towards us - they are not firing in any of the cases where Gale talks about his motives and desires to us, ergo, unless Larian is abjectly cheating to cover their lazy story-telling, he's on the level with us, completely. If you think he isn't, then you think that he is being allowed to deceive us without procing any checks and without our characters being
allowed to determine that he is being deceptive in any way... and why is that okay, here, when everywhere else, we have functional tools for determining such things? It's not about player characters and companions - we get insight checks to call Wyll's bullshit in many places; Gale does not proc any, even when we might have every reason to doubt or be incredulous.