DS has always said he learns "by voice," yet he also does well with visual material. He likes nothing better than studying any sort of guide where he can compare every possible combination of something - all the types of bears in the world, all the various spaceships, all the whatevers of the Star Wars series, etc. That, and watching tv, of course (documentaries are fine). And I suppose he is non-linear b/c he can see something and understand the big picture and make observations about things others overlook. But he's also linear, I think...

As for me, I always hated lectures because they went too slow for me. I'd much rather read the book. I'd usually take notes as a way to force myself to listen, or I'd doodle, or play word games on my paper. Interestingly, recently (in the last few years), I am able to sit through lectures and enjoy them. Maybe that's because in school, I had all these other things I liked to do, and now I am so grateful to be OUT that I'll sit through anything?

I am a little bit confused...

is non-linear = visual and linear = auditory?

What about kinestetic learners?

As well, many of the characteristics of the visual are the same that those of HG+.
Are HG+ people more visual than the general population?

This is my understanding;

Non-linear vs. Linear
Holistic vs. Sequential
Visual vs. Auditory
Right-brained vs. Left-brained

I once heard a memorable quote regarding this which stated that left-brained individuals usually have higher grades (as a group), but right-brained individuals usually have higher IQs (as a group).

non-linear = visual-spatial (right-brained) learner
linear = auditory-sequential (left-brained) learner

I'm not entirely certain, but I think kinesthetic would be a subset of visual-spatial. Under this theory, anyhow, there are only two kinds, not the many kinds under (Gardner's?) theory of Multiple Intelligences.

About giftedness, my understanding is that HG+ are likely to have superior skills with both left- and right- brained learning. Silverman notes that the kids who scored very well on IQ tests did very well with visual-spatial items - it sounds as though it would be difficult to score as high if one did not have visual-spatial strengths:

"I’d like to share with you how the visual-spatial learner idea originated. Around 1980, I began to notice that some highly gifted children took the top off the IQ test with their phenomenal abilities to solve items presented to them visually or items requiring excellent abilities to visualize. These children were also adept at spatial tasks, such as orientation problems. Soon I discovered that not only were the highest scorers outperforming others on the visual-spatial tasks, but so were the lowest scorers. The main difference between the two groups was that highly gifted children also excelled at the auditory-sequential items, whereas children who were brighter than their IQ scores had marked auditory and sequential weaknesses. It was from these clinical observations and my attempt to understand both the strengths and weaknesses that the concept of the “visual-spatial learner” was born."
http://www.visualspatial.org/Articles/intro.pdf

As has been discussed above, many people are a mix. For example, my DH is HG, and is strong with both left and right brain things, though if forced to choose he'd probably choose visual over auditory. So, to answer your question, I would say that yes, HG+ people are probably more visual than the general population - but then, they may well have greater auditory-sequential strengths as well - just plain "more"! Somewhere in her book, Silverman says that those who heavily favor only auditory-sequential learning can visit the world of the visual-spatial learner but cannot live there.