You have some work to do to show that the gorilla will "routinely" outsmart the PC in a way that affects play meaningfully, particularly as it's unlikely (but not impossible) that a gorilla will feature quite often in this character's adventures. It's just not a worthwhile consideration and doesn't have anything to do with the rules. It's instead to do with your apparent preference as to how a player should portray a particular ability score. You're welcome to your preferences and your table rules, of course. I just don't share them.
A PC with INT 5 is literally dumber than gorilla. I'm not sure how else you can spin that. It doesn't matter if the gorilla being smarter would "affect play" or not, you're RPing someone who is dumber than a gorilla.
Now, that might be because they're some kind of idiot-savant, but if you're portraying them as absolutely normal, or even brilliant, that's not just a "preference" or "table rule", that's ludicrous.
And it's exactly what the thread title is about - some people do sometimes take totally ludicrous and unreasonable approaches to character stats, usually, in my personal experience, because they essentially want their character be the most special snowflake possible, often using some bizarre mechanical angle, but can't stand to have that snowflake marred by the indignity (oh the indignity!) of actually being weak/rude/stupid/incompetent/clumsy/etc.
I'm not completely unsympathetic. I've felt the urge myself. But if you pick INT 5, you should RP INT 5, not INT 10, let alone INT 20. They might be more "special" than "stupid", but they're not going to be Sherlock Holmes, who is a peculiarly cerebral character. Wanting INT 5 to be Sherlock Holmes is like wanting STR 5 to be Conan the Barbarian (Arnie-style), maybe because you did a DEX build and dumped STR hard, but still want the visual image of the musclebound hulk.
Low INT doesn't have to mean stupidity, or low general reasoning. It certainly can represent a person with high general reasoning, but a lack of impulse control, or an inability to focus, or sit doing the same repetitive activity.
I don't think this is supportable, and you offer no evidence or examples to support it.
A lack of impulse control is clearly nothing to do with intelligence in D&D, period. Nor is "sit and do repetitive tasks" (which has never related to INT in any edition of D&D. These two sound very much like you're quoting from a source about not thinking kids who can't do those things are stupid, which is nice but irrelevant. Inability to focus would impact some situations where the INT stat is used, but not others.
I mean, as someone who has ADHD (and yes my dad did make the AD&D/ADHD joke at one point!), I have all the above traits, but I see no reason why that would mean a low INT in D&D terms, because D&D INT checks (including skills) aren't really about those things - not even really focus (I'm extremely good at finding things, for example, despite having these traits - which I'm pretty sure would mean a good Investigation check in D&D terms). If anything, I feel like those traits might penalize WIS, not INT. I know I'm easily distracted, and in D&D terms, that probably means worse Perception, and lack of impulse control would tie in well with failing WIS saves and so on.
I think thus, it would be fair to say that someone could have a low WIS because of those traits, even whilst being quite circumspect about social relationships and so on, and having sensible ideas about life.
But the only ways I'm seeing INT 5 as not just all-round bad at thinking if they're not good at thinking, but a massive memory box, or some other kind of idiot-savant.
EDIT - I've seen an interesting double-standard re: stats since I was a teenager, note. People want to be able to dump mental stats without the consequences of dumping them. Very few people will dump STR and try and portray a big strong dude, but the number of people who will dump CHA into the floor and then try to play Westley, or worse, basically Kenneth Branagh's Henry the Fifth is staggering. And yes, people who dump WIS and then want to play someone who is wise in all ways. And let's not even start on the staggering number of INT 10 geniuses out there. But you want to say you're agile, and you took DEX 10? People aren't going to let it go.
It's a pretty weird double-standard and not really present in a lot of other RPGs, I note. You don't get Vampire characters putting 2 dots in INT and then playing like they're Xanatos, typically.