They havent released this feature yet, but once they do you can make your own decals by making a .svg file in illustrator (or other "vector-software), and then upload it via the gt website. There are plenty of websites with .svg files of existing logo's, so you dont have to make everything yourself.
At the other side, i've seen a redbull and john player special livery. I wonder if the creators got early access to this feature or if they just made the logo's with the existing decals