You can find this filter through Filters → Colors → Value Invert

This filter inverts Value (luminosity) of the active layer or selection. Hue and Saturation will not be affected, although the color will sometimes be slightly different because of round-off error. If you want to invert Hue and Saturation also, use Layers → Colors → Invert.

Note that hue and saturation can be distorted quite a bit when applying twice this filter for colors with a high luminosity ( for instance, HSV 102°,100%, 98%, a bright green, gives HSV 96°, 100%, 2% after a first application of the filter , and 96°, 100%, 98% after a second application) . Thus, you should not expect to be able to apply this filter twice in a row and get back the image you started with.