I quilt with a Gammill longarm and have quilted several double knit quilt tops with no problems. I do not use a ballpoint needle when quilting these. (however, when I made clothing out of double knits I always used the ballpoint needles) I do like to use poly batting as it will help make the quilt a little less heavy after quilting. Also I would recommend quilting from the front of the machine so you can see where the seams are and maybe be able to bypass the heavy seams.
Here is a picture of a Lonestar doubleknit that I quilted.