Use a primer for plastics (you will find it at any automobile paint shop) and then paint it with normal body paint.That's the way modern plastic bumpers are painted and GTL side guards are not different
I'm not sure what material the plastic panels are made from, but it might be possible to use plastic bumper paint. I bought some for my Citroen BX bumpers - cheap to buy, vinyl finish and very hard wearing. Pull a panel off and take it down to your local paintshop to see what they suggest.