what's wrong with Zubrin's plan?
> not enough gravity.
what's gravity got to do with it?
ADDED: if you're going to tell me the gravity is necessary to hold onto the atmosphere, great, i'm actually familiar with that mechanism. i assert that mars' proclivity for losing the lighter atmosphere over millions of years isn't relevant, and that it'll hold onto everything important for Long Enough.
When the speed of molecules in the atmosphere (thermal, and thus dependent on molecular weight) approaches escape velocity for the body, the atmosphere starts to leak that component. Gasses like N2 (molecular weight 28) and oxygen (32) will "stick" to Earth where Helium (4) won't. Mars can hold CO2 (44) but not O2.
and only hydrogen and helium are currently prone to jeans escape on mars. the current dominant mechanism for atmospheric loss is the solar wind. which eats away ~1/3 megaton/century. mars has teratons of atmosphere remaining. and it can be added faster than that rate of loss.
(once warmed up, oxygen may begin to escape the atmosphere again. not necessarily faster than it can be replaced, though.)
Gravity got the following to do with it: Lower gravity makes it easier for the atmosphere to escape to space, especially since Mars has a very weak magnetosphere. Second, to have a human-breathable atmosphere you need not only the right composition, but also the right pressure, and to get that pressure on Mars you'd require an atmosphere more than 2,5 times the mass of Earth's. Where are you going to get it from? Even if it was available to you, using it to make Mars globally breathable would be an ENORMOUS waste compared to simply pressurizing domes and bunkers instead.