This filter is found in
Image>Filters/glass-effects/Apply Lens
After applying this filter, a part of the image is rendered as through a spherical lens.
Options
Parameters setting
Keep original surrounding: The lens seems to be put on the
image.
Set surroundings to Background color: The part of the image
outside the lens will have the Background color selected in Toolbox. This can
be a transparent background.
Lens Refraction Index: Lens will be more or less convergent (1-100).