Old thread, but to clarify, there are two ways. Have not played Forza but I have seen other games that have a "mirror" function...GTS doesn't have that per se.
First way is to go into layer options (?) and "duplicate on reverse". This will put a second decal on the other side, flipped if it can be flipped. Note that is is side-to-side, L/R. You can't use this to duplicate from the front bumper to the rear. The decals are separate so if you move one side the other side does not move.
The second method I use quite often is to place one decal, for example on the left side, using align to camera (don't think this works with align to surface). In projection options you can bump up the angle and depth to mirror it on whatever side of the car is behind. This works best going straight on from the side. I bump angle and depth to 10 and it will show in the exact same place on the opposite side, the right in this example.
This method is not good for anything that has text but is great if you are making an intricate design with stripes, patterns on images. The decal is flipped "backwards" and cannot be flipped as it is the same decal from the other side; any changes will be seen on both sides. Bonus that it saves you some layers as well.
You can also cheat a bit by not going straight on. Because this method will display the decal on whatever panel is directly behind, you could place the decal on a quarterpanel using an angle where it would pass through to the bumper. Or angle it from in front of the rear tire to behind the front tire. Frankly I've never used it this way but it is handy to note as it can cause some goofy shapes to show where you don't want them if you were trying to create stripes or color blocks or such.
For visual people, an example:
https://www.gran-turismo.com/us/gts...lery/all/livery/1554667/1/4900484857324406281
In the images below, the stripes are done using the second method as I wanted them the same on both sides. The stars and United States banner were done using the duplicate reverse method. They are in the same place side to side but as the text would be backwards I had to duplicate and flip horizontal so it could be read on both sides. The stars were placed using surface align as camera align would have caused distortion around the curves and creases, but duplicating on the reverse keeps them in the same position on the opposite side.
Clear as mud?