Cannabis is not a gateway drug. Anyone who says that is spewing out a lot of nonsense. There isn't a shred of legitimate evidence on the planet to support that. It's just rhetoric left over from the old “drug war” time.
In fact, there is a good paper on that. It isn't at my fingertips right now—it's a pity I didn't have the questions in advance to prepare for that—but there is good evidence that it is not a gateway drug. We'll just leave it at that.