Of course it's possible to get "bodybuilding-type gains" naturally.... people were doing before drugs even existed.
However unless you are a genetic freak you will not be able to obtain the combination of size. hardness and fullness that competitive bodybuiders have. Even those competing in "natural/tested" competitions often take drugs (I know this first hand)... not to the same extent as the "open" category and most of the time they stop some time before the contests. Some competitors really are natural, but they are the exception.
By bodybuilding-type gains I mean getting muscular and lean, having a physique that visually stands out. Yes that is achievable. It is my belief that 90-95% of the people can get a lean muscular look naturally if they do what is needed (5-10% are on the left end side of the bell curve and will not be able to achieve that look). But only about 5-10% has the genetic makeup to be very lean, muscular and big naturally.
Most people CAN get muscular and lean (I'm talking 5-8%)... shirtless they will look "like a bodybuilder (or a physique competitor)" but they might not look huge in a shirt.... they will look above average and athletic... even the best true natural competitors when they are lean do not look monstrous wearing a shirt.
Most people CAN look thick and big in a shirt, but without a shirt they will not be so impressive because they wont be super lean.
A guy who is 180lbs ripped will normally be 215 at a normal bodyfat level.... if you are 5"8", that looks pretty darn big.
I'm not saying that to be a downer, but to show you what is possible and what is not. The biggest I was in "bodybuilding" shape was 188lbs on 5'8"... and that was at an "off-season" weight of 217... not super lean, but still with some abs. Now a few years later I was able to get to 225-228 (Built for Bad pictures) in a similar condition to what I was at 217... so maybe I would have been 195 in contest shape. That is HUGE on 5"8" and lean.
What I'm saying is that pretty much everybody can get that "LOOK" but some will get that look weighting 165 others 220. But really that's what bodybuilding is all about... looking the part, not weighting the part.
Can "lighter training" work? Sure, I never said that it didn't. At the moment I'm personally supplementing my strength and olympic lifting regimen with 2 bodybuilding sessions based on pumping work, and it made a significant difference.The reason why I write more about performance methods is that it's what I personally like to do and like to coach. That's also what I have the most experience with. I would personally not training "bodybuilding" full time because that's not what I love. But it doesn't mean that it doesn't work.
That having been said, even the bodybuilders that I train, do a good amount of strength work THEN to their pump work. This is especially important for natural athletes and especially when trying to get leaner to help preserve muscle mass.
For example a figure girl that I trained who won the provincial championships while being natural (and it was not a tested show) did more heavy work when she was dieting down then when she was in the off-season. The result is that she didn't lose any strength, even right up to the show.