You can still use this.
With the antenna mounted on your bumper, the frame, chassis, and rest of the car will become your ground plane. You will be somewhat directional in transmission and reception however. Since it has a magnet, why not put it on your roof or trunk lid anyway?
Some truck stops DO have ground plane "kits". It consists of a small acorn style nut, with 3 or 4 radials sticking out of the sides. I've ran those before, heck I still have one, but I can't tell it does anything for me. It just made my SWR higher.
If you're using this for an actual CB, I wouldn't be as concerned about ground plane, as I would be the SWR. Although both are directly dependent on each other..
If you have the antenna mounted on a bumper that IS NOT metal, you will have a high SWR. The magnetic mount antenna ground by inductance from the magnet. So you just cant "glue" the magnet to the bumper, or anything else. It has to attract to it. I had a K40 CB antenna on my old truck, and I could CHANGE the SWR by adding and removing layers of thin plastic under the magnet.
If you want to talk, bite the bullet and stick it on your roof or trunk lid. If you're just interested in listening ONLY, you're fine the way you are. Just don't use the microphone. LOL..