Why do I get "Enable Gzip Compression" error with GTMetrix?
The cause for this is that Autoptimize uses a technique called "low quality placeholder" which will display a blurred version of the original image before loading the actual image. For example:
These images are SVG files (basically plain text) that are very small, usually under 1KB. While ideally these file would be served by the server in its compressed version, there is a technical limitation that prevents ShortPixel from doing this: a compressed version of this image cannot have the ETag HTTP header served. So we chose to have the files served with their ETag header instead of sending the compressed version of these files which are anyway very small.
Our suggestion is to ignore this message from GTmetrix as these SVG images are already around 1K and compressing them would only save a few bytes each.
As an alternative, you could just deactivate this option in Autoptimize and the GTmetrix warning should disappear: