I did a post a while ago about the Indian Palace which still deserves its place as the top Indian restaurant in town.
If you are looking for somewhere quick and easy, but still with great food, look no further than Samosa in Engelgasse. They have just reopened after renovating and are as good as ever. For lunch, they have a buffet with 3 or 4 dishes normally on offer. The price is really reasonable for St.Gallen: CHF 15 for vegetarian, or CHF 18 if you also have the meat dish.
And don’t be afraid to ask for the mango lassi – tastes great!