Gale is not another player - he's an NPC, and if he was presenting something counter to the truth with a goal to deceive the player, then yes, he'd be making a deception check; behind the screen perhaps, but he'd be making one, and it would at very least be checked against the player's passive insight, if the player did not give any signs of other mistrust on their own. You do not get to lie and deceive the players directly to suite your story-telling needs without providing them chance to pick up on the fact that something is off.
If Gale were player actively trying to deceive another player and hide their motive, and the other player thought they might not be completely on the level, then yes, they make either a deception or persuasion check - they do not tell the other player which they are making - and it's a contested check. If the other player's insight wins out, then the other player should reveal which kind of check they were making, and any other relevant information that they feel might be giving them away, if there is more to the picture. It's not mind-reading or brain control, and it's usually adjudicated to be a reasonable degree of give and take, depending on the stakes being questioned.
In a video game, we can't do that - but more importantly we cannot, in most cases, even suggest through dialogue that we don't altogether buy what they're telling us. We are Forced at nearly every turn, to blindly eat up and trust everything they say and everything they tell us, and then, afterwards, react with shock like a kicked puppy when it turns out not to be true, and it's frankly infuriating. The companions also always get the final word on this, and are usually smarmy and condescending about it - all written by the same hand, irrespective of their individual personalities, more or less.
The most prominent point where you can attempt to deceive a party member, and have it fail miserably because she just waves the tadpole at you and reminds you that she, and everyone else, know everything, is when discussing your dreams. Unless of course she's lying about that - again, we don't even have the option of mistrusting her assertion, so we must presume that it is completely true and honest.
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I agree that nothing seems disingenuous about Gale's delivery; he's clearly confident, learned, but has the kind of personality that is prone to overconfidence and over-estimating his understanding at times; he is proud of his academic achievements and his abilities, and that pride can become a harmful flaw very easily, but most of the time he's quite happy to share his knowledge with others and help educate - he isn't covetous or hoarding of his knowledge, as some academics are, and when others display understanding past his expectations, he's quick to compliment them on it. He views himself as a pragmatist, and when his options run dry he takes risks upon himself, rather than bringing direct harm or difficulty to others; rather than steal your magic items or consume them on the sly, leaving you vulnerable and without things you believed you had, he accepts the sacrifice of staking his own soul with Raphael - frustrated with the party, sure, but confident in his own ability to manage the situation and hopeful still of coming out on top of the deal, somehow.
Nothing seems to indicate that he's not a good and decent person at his core, and nothing that he does or says is particularly dishonest or disingenuous - tactless at times, and a little rude at others, but easily the best of the bunch presented to us. Which will make it all the more aggravating and dissatisfying if Larian turn around later and try to heel-turn him into being secretly and evil megalomaniac who is just good at pretending towards geniality; because they would have god-moded his ability to deceive us unfairly, and their story-telling was too crude, lazy and poor to handle it in a more satisfying way.
I'm taking things at face value because ion other situations, when characters lie, cheat or deceive us, we get insight checks, and here we aren't, so clearly, if our tools are working correctly, they're on the level. If Larian later turn around and say "Oh, we disabled your tools back then so they weren't showing that, haha, isn't that cool?" I won't laud it as skilful story-telling, or clever deception by the writers - I'll just be put off and dissatisfied, because all they will have really done is breach the trust between the viewer and the story-teller, to the detriment of the story experience.
I like Gale; to me, he seems like a stand up person, a good guy and a perfectly reasonable individual who has the best for everyone in mind, to the extent that he pragmatically can. He's a guy who fucked up, majorly, and is carrying that burden - quite literally in his chest - and hasn't necessarily conquered his hubris yet either, despite that, but otherwise is clearly the best and most heroic individual out of our current companions, by a long measure (with caveat for the liklihood that Shadowheart is brainwashed and is an otherwise heroic Selune worshipper, who is also probably Selune's chosen, and Shar's chosen, and the living avatar embodiment of the proof of the darkmoon heresy, destined to bring balance and unity to the twin goddesses. Tell me I'm wrong, I dare you)... provided that the tools that Larian have given us are working correctly, as Larian tells us currently that they are.
