This looks like a new keyboard resizing feature introduced since Gboard 13.0.
According to 9to5Google Android Police - Gboard for Android preps an easier way to ‘Resize’,
“Resize” would be another shortcut in the upcoming toolbar/panel redesign. It starts by letting you adjust the height of keys and the panel by dragging up/down from the top or bottom edge.
[...]
What’s new (as seen in the screenshots below) is how you can move the resized keyboard (center button) up the screen so that empty space exists between the bottom edge and the system status bar. [...]
(Emphasis added)