You can add gravity pretty much into any functon as long as it gets executed, but a good place to place it is your main function. From the manual
Code:
ph_setgravity ( vecgravity );
and for the vector you want to have a downward force applied along the z axis, and in A6, -386 I believe is equal to earth gravity.
Code:
ph_setgravity (vector (0,0,-386));
Once you got your gravity, then you need to create your vehicle body either through WED using an action, or using ent_create. Then you apply your forces friction, mass, damping, etc. to it. Check out the manual and look around here for examples, and BTW, if your new to A6, this probably isn't the best place to start.
You might want to check out the templates, I think there is a template now for vehicle physics, but Im not sure, I never used them.
PS: You need the Pro version of A6 to build a vehicle using physics...