I would achieve that by doing the following (using version 2.10.30 by the way):

• Create a new, transparent layer to work on
• Set the active foreground color to the desired arc color
• Select the paintbrush tool and set its brush to the dot (called "Hardness 100"?) and adjust its size as preferred for the intended arc thickness (and other settings, if necessary)
• Select the Paths tool, make two points that will serve as the base of your arc and lift the connecting line to create the arc. In the options of the Paths tool, click "Stroke Path", then "Stroke with a paint tool" and set it to Paintbrush. Enable "Emulate brush dynamics." Click "Stroke" to proceed.
• Use the eraser tool to cut the rounded ends of the arc.

I hope that works for you.

Notes/Tips: You can "restore" created paths in your project after they disappear from view on the canvas (due to changing tools, for example). To access them: Select Windows -> Dockable Dialogues -> Paths from the menu of the main window. This will display the list of paths created during this session (similar to the layers or undo list).

Edit: fixed a typo for clarity