Closing the REPL pane or Git/GitHub pane

I prefer to use keyboard bindings when I can. One binding I haven’t been able to figure out is closing the REPL pane (I think it is called a pane but I could be wrong). I can do it with the mouse by clicking on the down arrow in the semicircle above the name REPL. Once it is closed you can open it again with -J -O.

Similarly -9 opens the git pane to the right but I can only close it by hovering around the boundary until the right-arrow in a semi-circle appears.

Can someone help me expand my Atom knowledge?

You’re almost correct – the REPL is an item in a pane, which itself sits in a dock. As such, you can use the Window: Toggle Bottom Dock and Window: Toggle Right Dock commands for that.
Arguably, Ctrl-J Ctrl-O should hide the dock if the REPL is focussed, but I’m not 100% that’s desireable.