For folks with celiac disease, eating out without getting glutenized is tough. It seems like everything is suspect.
Subway sandwich shops, however, at least make an effort to not lace everything with wheat. Go to this link and scroll down to "Information for People with Food Allergies."
Of course, all the buns are out. Don't order a sandwich or BYOBun.
The salads, however, are pretty straightforward. With the exceptions of the the Meatball Marinara, Seafood Sensation, and Sweet Onion Teriyaki Chicken, all the other salads are safe if you tell them to hold the croutons. Meat, cheese, veggies: it's all good.
As for the salad dressings, the only one with wheat is the "Atkins Sweet as Honey Mustard." All the other ones are free game and GF. Enjoy!