Hm, but that quote you quoted can't be quite right, can it (although I acknowledge that you were making a point mostly about physiological effects of the trip, not the atmosphere)? Atmospheric pressure (or density) can't just be a function merely of surface gravity. Just from observation:
- There's Venus, which is about the size (and roughly the surface gravity) of Earth, but has 90 times the atmospheric pressure..
I already explained this briefly in another post. Another trait of Venus' atmosphere is that at its surface the pressure is equivalent to being 900 meters under an ocean in Earth. Even standing on it without equipment would get you crushed - but that is not before you get incinerated by the C450+ temperature that could cook a pizza in about 6 seconds - let alone a human body. This is because of its composition, with a very dominant presence of CO2 (over 96% of its atmosphere is this chemical compound alone).
Titan may actually be a more valid candidate than Mars in that it receives most of its heat from Saturn tidal gravity waves, and so it's a whole different beast. We don't know enough about it yet to tell.
The bottom line is yes, indeed, you can make Mars atmosphere denser by using GHGs (I mean, in theory, not that we can embark on such an enterprise at this moment, but let's imagine we actually can) - However, for the atmosphere to get there, and due to the distance from the Sun (and the amount of heat that Mars receives because of it), the atmosphere would become toxic for humans in the process. You can't have it both ways, either you have an atmosphere with Earth's composition that is too thin and cold for humans to be able to breathe it safely, or you have a thicker, heavier atmosphere with a high content of CO2 or other similar chemical compounds to reach a pressure similar to Earth, but then it would be toxic for humans. If Mars was a bigger planet, its own gravity may be able to make up for it, requiring fewer GHGs to get the atmosphere to a desirable pressure before it becomes too toxic, but that is not the case.
In a nutshell, this is why planets need to be in the Goldilocks zone to be even candidates at all. In theory, if we had the technology necessary, we could terraform any planet within the Goldilocks zone to have a breathable atmosphere by altering its composition in a sustainable way related to the heat that it receives from its local star.
Neither Venus nor Mars are in the Goldilocks zone, and they will never be planets that humans, as we know them, can stand on their surface unassisted by technology to survive for even more than a few seconds.
Edit: Wanted to add that of course, we're always speaking about humans. As species, we have evolved over hundreds of thousands of years from other species, and everything in our physiology, our bone structure, our muscular tissue, our senses, every tiny cell in our body has evolved and grown "cozy" around surviving in Earth's environment. Any tiny alteration to that would put us at a heavy evolutionary disadvantage. That does not mean that Mars or Venus could not be "Terraformed" to sustain other kinds of life comfortably that may evolve acclimated to those different rules - just not human life as we know it.