Even though the tension may be ok on other fabrics, you may still need to adjust it separately for the terylene. My machine also has a foot tension that can be adjusted so you may check your manual to see if you can adjust yours. Gathering means that the fabric is being pulled as it feeds through the machine so either the thread tension is too tight or maybe the foot tension is pressing down too hard. If it’s not the thread, consider using a walking foot to see if that helps a little. If there’s still an issue, hold your fabric taut from behind as it feeds through to prevent the gathering or sew it with a strip of tissue paper on top. I don’t know why the tissue paper works but it seems to help when everything else fails and I’m working with annoying fabric. when I finish my stitch, I just rip the tissue paper off. I hope this helps! 🙂