Hi there,
I tried drawing a proper circle usign the ecclipse selection tool, and the circle looks like jagged on the selection line as shown here:
How can i draw a circle that is round like paint?
Gimp is a raster (bitmap) editor and works in pixels. That boundary you see the edges of the pixels. The line shown by the ellipse tool does not really exist, only in the Gimp Interface.
Back to your screenshot, That is very-very small and is much magnified. I duplicated here. 17 x 21 pixels. and 2300% zoom Like this:
https://i.imgur.com/pOCYyka.jpg
To make lines appear smooth anti-aliasing is used, so make sure that is enabled. You can fill a selection just by click-n-drag from the swatch into the selection or you can use the bucket fill tool or for something that small you could paint it in - up to you.
That gives this:
https://i.imgur.com/fUVbbFj.jpg
Still looks jagged to you. Consider it in context with a more regular size canvas.
https://i.imgur.com/kT72Wna.jpg
Want just a circle? Then use
Edit -> Stroke selection like this :
https://i.imgur.com/tcvieYb.jpg One proviso - use an odd number of pixels for the line width, otherwise you loose the anti-aliasing ( much like MS Paint
https://i.imgur.com/FetcwSX.jpg )
That is a very small circle, probably 15 pixel diameter.
Gimp is a pixel editor. So the selection follows the pixel boundaries. That is why it is jagged.
You don't say what you want to do with the circle. One thing that can be done is stroke the circle.
One way is just to use Edit > Stroke Selection
Or you can first convert the selection to a path Select > To Path The path is not initially visible, so go the Path Dialogue and click on the eye to make the path visible. Then Edit > Stroke Path
Another way is to make the selection bigger. On a 200 pixel diameter circle the jagged edges are barely noticeable.