Shift gates are those 1-3 cut down teeth you see on outer chainrings. The gate makes it easier for th chain to derail and move sideways without having to climb up over the top of the teeth. Gates make it easier to shift under load, and usually ensure that shifts happen at that specific location, so that the timing where the chain meets the inner ring is more predictable.

Gates are what puts the Hyper in Hyperglide -- or maybe it's the glide.

Anyway, innermost rings never have nor need gates because the chain is always lifting up off the ring (or dropping from the top) during shifts.