@Typical @Vertigo, sorry this got kinda long!
So GUTS+ does heavily rely on GM interpretation, but GMs are also encouraged to 1) favor the characters, 2) only ask for rolls on things that could have consequences, and 3) take the situation and the characters into consideration when interpreting. Also, more amorphously, the interpretation is not the average value of all the dice.
So in Duncan's case right now, he's trying to kick open a door that opens inward by kicking near the door's latch, i.e. the solid outer frame of the door. In my reasoning, even as a strong basketball player, it would take a miracle to break the sturdy door frame (by which I mean the piece that the door's hinges are connected to and has a lip that prevents the door from opening regularly) and let the door swing outward when the door is designed to open inward. Instead of simply saying it is impossible (which it's not—if super lucky, he could break the knob/latch mechanism or if he used another method, i.e. kicking the middle of the door that's hollow or ramming through that hollow section), I asked for a roll because there's a possibility that if he rolls low, his kick could land wrong and give him a minor injury on his foot. I probably should have communicated the low possibility a little bit more clearly... If Duncan has only 1 Gumption, he would not have been given a roll if he tried to kick the door that way, but his 2 Gumption gave him a slightly higher than 0 chance at getting lucky.
The outcomes, positive or negative, depend on what's being done. Since GMs can't plan for everything, like this situation, I had to think of what could happen. If something truly comes out of left field, the negative impact is often a negative Status Effect, for example "Duncan kept trying to kick the door open the same way, but beyond bending the door handle up, he just gets frustrated. Duncan gets the Frustrated status, which reduces the value of any one die rolled for Thought rolls by 1." Or something like that.
As for the number of dice thing, here's where the "not an average" idea comes in: if someone's Gumption is 6 and they're trying to jump over a 10-foot gap over a ravine and they roll 1, 3, 4, 6, 6, 5, then that skews heavily toward success
plus they get a double if they want to use it—or if that person was an expert long jump competitor, then it would be a Positive Impact because of the character and the situation. If that same character rolled all 1's and 2's, then the doubles might help them, but there's always a chance in life that you trip. If that same roll were applied to say moving a log out of the way of an entryway, it would most likely be a Positive Impact because the task is easier. As such, die rolls could be compared to divining interpretations, which is why players are also encouraged to reason with the GM and make arguments for success. For example, I've never personally tried to kick down a hollow-core door from the inside, but if you have and it's actually much easier to do than I think, then you can say so and argue for it! It's kind of a checks and balances thing when the result is gray.
I hope that all makes some kind of sense. I'm also not an expert in statistics, which is why I designed it with so much GM power. Rolling doubles was designed to help the odds a little bit and it gets more likely that you'll roll doubles the higher your level, but the GM interpreting outcomes and players arguing against interpretations they disagree with is still pretty important.