Sunday morning rantings:
The CG is a 3 dimensional point in space of the front-to-back, side-to-side and top-to-bottom balance of these weights, somewhere around the rear firewall between the seats above the center tunnel. The length and width balance points can be readily measured (4 corner weight), but the height of CG would be somewhat more difficult to get. You could use the aviation build method and weigh each individual component, then do the math to determine how it moves the CG from a datum reference point. (Too complicated to think about this early in the morning) Fuel tank level and driver/passenger occupancy/mass will shift the CG from its 'empty' position and will affect things like suspension calculations.
Keep in mind the relative CG point on the roll axis is different over the front suspension versus the rear suspension. The imaginary line that goes through the CG front-to-back in angled upward toward the rear of the car, since the drivetrain mass is greater and 'taller' in the chassis than most of the rest of the car. So the front suspension CG is shorter coupled to the roll center than the rear suspension, which feels the effects of the higher CG with a longer 'lever arm' acting to produce body roll. Keeping the rear body roll under control is important to setting up the suspension for the Goblin to handle effectively, but it is harder to achieve with the McPherson struts. Roll center changes are more difficult to make that can have an adverse affect on other suspension factors.
Suspension calculations are dependent upon the position of the CG for roll center, weight transfer, dive/squat and several others. With the Goblin being light weight and short wheelbase, it is easy to shift the CG with relatively minor changes in weight and location. If someone can calculate the 3d CG for an empty Goblin it could benefit those who race their car or are tweaking the suspension. Ok, I'll stop now, think I need a nap.