Most all new cars are 'bottom breathers' - they pull air from the high-pressure region behind the front fascia and ahead of the front wheels. GT's are kinda funny - while they look like bottom breathers, they are 'front' breathers - they pull air from the front of the car, under the front bumper.
While this may look like a good design at first glance, it's not as efficient as a bottom breather design. A fair amount of air 'spills' off the front end as it cuts thru the air and literally 'misses' the cooling opening. An air dam does exactly that - 'dams' the air from spilling off the front of the car and piling up under the front of the vehicle, causing lift.
Cars are funny when it comes to this. Low velocity air has a higher pressure than high velocity air - it's this function that provides aerodynamic lift to a wing. Unfortunately, the underside of a vehicle is a great place to 'catch' air, thus slowing down airflow. Meanwhile, the air is flyin' over the top surfaces at great speed - the result?
lift.
The effect can be significant - several hundred pounds of lift at 100 MPH on a poor aerodynamic design. An air dam prevents air from piling up under the car, thus reducing downforce. I did some experiments with an Escort on stock rim protector tires back in the 80's, and it was possible to make the car 'stick' like it had 50 series tires with 80 series rubber - at speed.
The car looked funny, tho. To get an air dam to work well, it has to be 'in the weeds' - no more than 2" off the pavement. At curb height (over 6") they are largely ineffective. In addition to the dam, side skirts are necessary to prevent air from spilling into the low pressure area under the car - the effect is tail lift. Racers get around this by putting the entire car 'in the weeds' - not much air can get caught when the belly's scrapin' asphalt. Rules cops spend a good part of their time checkin' vehicle height restrictions - they have a big impact on how the car performs, often much more than the engine mechanical restrictions in SCCA class racin'.
So - back to your question. Depending on how you config your airdam, you'll actually increase the amount of available air for cooling at the FRONT of the car. If you restrict the opening, you'll decrease it. Distance to the pavement will have a big effect on this. GT's are limited as to how far down to the dirt you can go with a dam - there's a LOT of front end overhang to contend with. Chances are you won't be able to go down much more than the stock chin - an inch or two, tops.
If you wanted to build a bottom breather, the hot ticket would be to build a deep airdam about 12" ahead of the front axle centerline. This will keep air out of the chassis and won't get bunged up in daily driving. From there, I'd cut a 12" x 30" opening ahead of the airdam to get air to the radiator. Rule of thumb on the opening is roughly 1.5 - 2X the surface dimension of the radiator to minimize pressure drop.
Just keep in mind that you can't get much more radical in your envelope dimensions than the stuff you've seen on the OGTS website. The front overhang and cooling requirements won't let you.