We can distinguish 5 kinds of legendaries :
Among those 5 types :
Kadala can give you only Regular ones and Torment-only ones (since patch 2.0.5). Since you can gamble in every difficulty, you can hypothetically obtain a Torment-Only legendary in Normal difficulty by gambling (though it requires some luck).
Kadala can neither give Plans nor Potion (since she does not sell any of those). She can't neither give Cache-only legendaries :those are still reserved as a reward for completing bounties.
Short answer : If you want to know if you can obtain a legendary at Kadala, just look in which category it belongs. If it's not a Plan/Potion or a Cache-only legendary, you can get it at Kadala.