You can't just teleport in the game you need to either code it or use a trainer. Trainers cannot teleport you inside a building. The way the game functions; it teleports you to another dimension inside the game every time you enter a building. Take note of the x y z coordinates of the casino, the z altitude is 998.9694 meters above ground. This is why we referred to interiors as heavens. They are located in the sky. But teleport there using a standard trainer from outside and you will just fall to your doom. Just as the real heaven is another dimension the GTA heavens are too. You must use a Cleo script or edit the main.scm to get there. The only "correct way" to teleport there using a trainer would be to be inside the building or another building that exists inside the same heaven. To my knowledge there are 15 heavens. I believe you can use the Canton Gym to teleport to the Casino because they are both interior 1.
JAB
I think there are more heavens. At least 26, I've heard and explored myself that there are some weird interiors and there is a hidden safehouse too where you can save your game; the only bug there is when you exit by the door, you fall and then get back to the house.