I'm using the Gedit text editor.
For a single line, we can simply press Backspace to remove the indentation on that particular line.
How can I select several lines and unindent them all?
You can remove the tab space from multiple lines by selecting those lines and pressing Shift+tab. Shift+tab is used to give a reverse tab, i.e. tab in the opposite direction, so this should work almost wherever you want to reverse tab.