@Poochie - there are two "models" of tagines: serving tagines are elaborately decorated and cooking ones, which has less decorations on it. I have no issues with putting mine directly on my gas cooktop - just warm them up slowly and don't go over medium heat. If you research them, they are traditionally set on top of charcoal fire in another clay pot. I own 2 of them and use them several times a year, as I like Moroccan food. Here's a link to a good source for both "models" of tagines.
Moroccan Cookware | Treasures of Morocco
No different than the Donabe pots that many of us here have for cooking rice. Goes right on the gas stove.
I also have Bulgarian "clay pots" for making stews, which typically go in the oven (cold) so everything heats up uniformly.