Guetzli Converter is a simple GUI of the command line tool Guetzli developed by Google. You can find the official project on Github: https://github.com/google/guetzli
Guetzli Converter just simplify the process of creating a Guetzli encoded JPEG by providing a graphical interface.
Guetzli Converter's batch processing make it possible to run Guetzli multi-thread. Unlocking you much more performance if you have a bunch of files to convert.
Guetzli is a JPEG encoder that aims for excellent compression density at high visual quality. Guetzli-generated images are typically 20-30% smaller than images of equivalent quality generated by libjpeg. Guetzli generates only sequential (nonprogressive) JPEGs due to faster decompression speeds they offer.