The Check button (next to the glass for searching) does not have tooltip, and when pressed on have to spend some time to figure out its purpose. The main change is the color of the headerbar (it switches to blue).

Because the main change is the color of the headerbar, one may not notice that a checkbox appears next to each project.   I noticed after 4 or 5 tries going back and forth between the check button and the cancel button in the blue headerbar.

After some other tries I noticed that a another bar appeared at the bottom.

Probably a victim of using a big window for gitg, so changes in the UI are more spread.

Anyhow, a tooltip may help. Something like: "Select projects to remove" or better.