Rice, in its pure form, is gluten-free. The problem you can run into with rice is the way it's cooked, or rather what it's cooked in. Many restaurants use chicken stock to cook their rice. Some chicken stocks contain gluten, so any rice cooked in such stocks is not gluten-free. Always ask about how rice is cooked when dining out, unless you are ordering rice off a gluten-free menu. I've run into rice cooked in gluten containing stocks when dining out more times than I can count. Since I always ask about rice before ordering, I've never unknowingly ordered any that contain gluten.
Another way to find gluten in rice in restaurants is in sauce that is added to it. Some times it is possible to order safe, plain rice with no sauce and some times it's not. It depends on whether the rice comes in a bag already in the sauce or if sauce is made in house and added to plain rice. Even when the latter happens, all the rice and sauce might be mixed up before each shift, so there is still no way to order plain gluten-free rice. It is quite annoying not to be able to have something as simple as rice when dining out, but it's often a problem. Never assume anything is gluten-free when dining out.