Guetzli is described as 'JPEG encoder that aims to improve the web experience through highly optimized image compression that still yields great image quality' and is an app in the Development category. There are four alternatives to Guetzli for Mac, Windows and Linux. The best alternative is MozJPEG, which is both free and Open Source. Other great apps like Guetzli are Flawless (Paid), libjpeg-turbo (Free, Open Source) and libjpeg (Free, Open Source).
Flawless is a Mac desktop app to facilitate iOS development by helping you compare between mockups and implemented designs in real time. The tool includes overlay and split comparison modes with built-in iOS simulator to help ensure designs are as expected.