I just tried to find the option to display the line numbers in the code editor. The german preference dialog hides this option in Code-Editor -> Zeilenzwischenraum. Zeilenzwischenraum means line spacing, the space between two lines, while Line Gutter should be translated with Randspalte, meaning a vertical space (column) at one side of the text.
Außenrandbereich is surely better than Zeilenzwischenraum, though it is not specifically a vertical space, more like a frame.
Personally I would prefer Randspalte but both translations are acceptable.
P.S.: Randbereich is synonymous to Außenrandbereich and shorter. A border (Rand) is usually at the outside (Außen), isn't it? :-) Almost a tautology.