Pixelitor is an open source image editor

Pixelitor is an advanced image editor with support for layers, layer masks, text layers, multiple undo, blending modes, cropping, Gaussian blurring, unsharp masking, histograms, etc.
It has more than 110 image filters and color adjustments, some of which are unique to Pixelitor.
Some of the gradients are unique to Pixelator.