Unlike North Indian cuisine, South Indian cuisine is a whole different thing. If you haven't experienced it, start with the Masala Dosa, a huge gluten-free, crispy crepe filled with spiced (but not spicy) potatoes. The sambar soup that comes with the Masala Dosa can be a little spicy, but they also serve it with a cooling coconut chutney. Cheap, yummy, and very gluten-friendly.
Karma is my second favorite Indian restaurant in Philly. Since it is in Old City, I don't go there as much as I would if it were closer. Their lunch buffet is wonderful.
Saffron Indian Kitchen is my favorite Indian restaurant in the Philly area.
The bad news is that Saffron Indian Kitchen is in Bala Cynwyd, which means that it is not in downtown Philly. Like most Indian restaurants, it is very gluten-friendly.
A wonderful, white tablecloth Indian restaurant. They have a great lunch buffet that is all-you-can-eat (I try not to). Veda is higher end, but worth it.