New Answer to question after edit:
For this answer, I'll be basing the creature off of this image (drawn to the description of OP) of the creature in a 100% upright, back straight position.
A side view should look somewhat hunched like a gorilla, in order to facilitate the type of movement OP has described. Note that you can't actually have the lower body carry the majority of the weight if you want a super strong upper body, since the upper body has to have extensive muscling.
Since OP has said that the majority of the fighting will be:
fighting 1v1 and small group fighting they're most likely to be coming
up against an array of blade types and short hand-held weapons (short
swords/ maces/ rapiers), as well as larger two handed weapons
(warhammers, greataxes, bastardswords)"
the ideal armor would have to be chain mail armor, but only on the body section, and not for the arms (we leave the chain armor off the arms because if it's a one piece it may restrict shoulder movement more than his muscles already do). Chain mail provides good mobility and flexibility while giving decent protection against slashing from blades. The fighter will still be weak to thrusts from rapiers and the like, so he'll have to parry those in order to prevent damage. Chain mail also doesn't protect against the war hammers or great axes, but it won't be a problem if we give him the weaponry I'm about to suggest.
First, take a look at the following two diagrams.
If you look at the first diagram, you can see that point A and C represent the relaxed and maximum raised positions of the arm. If you look at point D on the spec, you'll see that I've tried to represent the extensive muscling in order to provide massive arm strength. This extra muscle will get in the way in terms of raising his arms, preventing many overhead blows from being possible. Compare it to the human diagram, where there isn't that much muscle on the shoulder area. Since we can already see that he wouldn't be able to move swiftly on his feet based on what OP has said, the weapon must be able to be used while mostly stationary, while making up for the weaknesses in his armor. Since the weaknesses in his armor can be overcome by parrying the incoming attacks, as long as the weapon allows for good parrying abilities, everything is good.
To this, I suggest an unconventional weapon. Use shields with plated arm protection. Take a look at the arm section I've drawn here:
This drawing here depicts what I mean by plated arm protection. By only plating the outer section of the arm, we maintain maximum mobility while still allowing the use of his arms to defend himself.
I say to give him (viking style) shields because the shields allow the species can still maintain his mobility while having good defense and offense through shield bashing. With shields, he is able to push away many thrusting attacks, or use his arms to deflect attacks. While even heavy plate armor has trouble dealing with maces or war hammers, a shield can designed to handle the punishment. Shield bashing with his immense strength can cause more damage than a punch as well.
But how would he do this? With what "style"?
Well, there's really no "style" that would work here, but here's what he can try to do.
Taking advantage of the somewhat wide open flat areas that they'll be fighting on, and his immense reach, I suggest using a karate style blocking and punching system for his upper body. (Disregard his lower body since his legs are too short for good karate kicks) This is because his immense reach means that he can intercept attacks using his forearms earlier than most other fighters. The plated arm protection allows him to parry Rapier thrusts, or longer thrusting weapons (spears, pikes etc). I also suggest that he can use those shields deflect attacks instead of just using his forearms. By punching towards the larger weapons (great axes, war hammers), a well aimed punch can completely throw the opponent off balance by reversing the movement of the heavy weapon. Furthermore, because shield bashing does not require overhead blows, we can avoid the limitation of his muscles blocking his arms from being raised.
To take advantage of his "speed bursting dash with arms" ability, he can hold his shields and use them while dashing, and place his shields flush to the ground while dashing (use them as hand-shoes). By propelling forwards, then thrusting his arms out, he can put his entire body weight behind his shield bashes, allowing for even more power. And since these dashes are only short distances, this allows for "chained" short dashes between multiple enemies.
I imagine the dash would look something like this (Arrows represent the directions of force):
As you can see, the first step would be to place the shields against the ground. The second step would be to throw himself forwards using those shields and launch himself into the air towards the target. Upon landing closer to the target (the completion of the single step dash), he can thrust a shield (or two) to cause damage. This easy 3 step process can be repeated back and forth between different opponents if required.
In terms of offensive attacks using his legs, he can attempt a gymnastics move type attack. By using his arms as pillars on the ground, he can throw his legs forwards as a kick - kind of like this:
Normally, I would ask you to clarify your question. Do you want to know which weapons would be best for this creature, or are you looking for a specific style? You're asking over 5 questions in the body of your actual question, and usually, this would already make me put in a vote for closure.
However, since I can answer all these questions in the same succinct answer, I won't make that request.
First of all, you haven't described a creature that's "a little bit like a
gorilla human" - You've basically described an upright chimp, minus the feet dexterity.
For this answer, I'll be using the image of a chimp-like human in my mind.
Now, the questions: However, given a level of medieval technology, how would this type of creature fight with weapons? Which weapon would be best for it, and what kind of fighting style would it use? In addition, what composition of armor would offer the most benefit for this creature to fight? What would offer the most protection for the fighting style, whilst still maintaining the least restrictiveness? What would be the most advantageous technique to fight by harnessing its strengths and minimizing its weaknesses?
Given a medieval technology, assuming that this creature has attained at least human level intelligence, it would not have a specific weapon type to use at all. The humanoid build and intelligence would drive the creatures to use whatever weapons fit the situation. While you could train a specific member of the species to be god-like at a particular weapon, it's more likely that the entire species will operate more like the human army - swordsmen, archers, pikemen, the lot.
Even humans don't have a "best fighting style". Every style has its own weaknesses, and at the end of the day it's the fighter that beats the other fighter, not the fighters style. Get this through now; There is no perfect style. Different members of this species will probably end up training in different styles, simply because at a human level of intelligence, choice is something crucial to the species - that means it'll be impossible for them to ALL train in the same style, since at some point one or two of them will branch out and more will follow. This also means there is no "Best technique" since every technique has a weakness. A person who has a single basic kick practiced 1000 times is more dangerous than one with 1000 different kicks practiced one time each - an old proverb (I don't remember where it came from) that can be interpreted as "even the most basic of techniques, if practiced enough, can be deadly."
Based on the above two answers (and further details you've provided), we can say that there is no best armor for this creature to fight. You've gone and asked a question with no answer. The armor composition depends on what kind of weapons the creature is facing. Plate armor against blades is standard, mail/leather for archers, etc etc. Again, there is no best fit.
TL;DR: You've asked the wrong questions, mate. Whatever you're doing in your story, if you're working with humanoid like creatures, can branch off our history. They can essentially use or make use of a large majority of our fighting styles, armor, and weaponry. There is no answerable "best fit" for any of your questions in which a weakness cannot be found. You're better off using some creativity and mixing styles (if you want something that isn't completely the same as human styles). Note that styles often come with weapons training specific to that style, so you may want to use those for your warriors.