Go to the Channels panel.
Hold Ctrl / Cmd and left-click the Gray channel to make a selection.
Press Shift + Ctrl / Cmd + I to inverse the selection.
Go to the Layers panel.
Click the Create a new layer button.
Make sure the foreground color is black.
Press Alt + Backspace to fill the selection with black.
Press Ctrl / Cmd + D to deselect.
Delete the ...