This trick is used to get rid of undesired squares (usually) on the edges and sometimes on the object surface (because of high compression etc).
1. Channel > Channel Combiner. Set it to “RGB to YUV”.
2. Blur & Sharpen > Channel Blur. Blur one of the channels (e.g. green channel).
3. Channel > Channel Combiner. Set it back to “YUV to RGB”.
Enjoy the effect 🙂