For a clean result (uniform thickness, smooth and clean edges):
Use the Ellipse select tool to make a selection
Select>To path
Select>None (otherwise step #4 will only create half of the line)
Edit>Stroke path in "Line" mode
Simplifications
If you do several circles, you can do a selection with several circles in step 1 (shift click for additional circles), and execute steps 2-4 only once.
For thin lines (3px or less); you can replace steps 2-4 by a Edit>Stroke selection (but this is ugly on tick lines)
You can of course do both together (many thin circles)
For a clean result (uniform thickness, smooth and clean edges):
Select>To path
Select>None
(otherwise step #4 will only create half of the line)Edit>Stroke path
in "Line" modeSimplifications
Edit>Stroke selection
(but this is ugly on tick lines)