The job of a gate is to "hold open" / "pinch closed" a channel with an electric field. The closer to the gate, the better the hold/pinch. In ye olde days, you'd slap a gate on the top of a channel and call it a day. Every part of the channel was close enough to the gate get a good pinch.

Then everything shrunk and smaller channels wound up needing stronger pinches to completely shut them off. Instead of slapping a gate on top and calling it a day, they raised the channel into a fin and drizzled the gate over 3 sides so that it could pinch from the left and right, not just the top. Those are FinFETs.

The next step is to have the gate on the bottom, too, so that it can pinch from all four sides. The channel literally goes through the gate, which surrounds it on all sides. Those are Gate-All-Around FETs, or GAAFETs.