Yes, that's the plate. I had hourglass gaunts without it (GDFB). The movement was okay, but it would gap when making a fist. When it doesn't gap, the movement isn't acceptable.
If I remember right, the SCA gauntlets have to be "grounded", meaning the finger plates when grabbing a weapon touch the weapon. Historical gauntlets were mostly flat and didn't do that, as it makes grappling and fine weapon manipulation more difficult.