What is GZIP compression?
GZIP compression is a way of compressing files so that they transfer faster. GZIP compression means your web pages load faster for website users.
The server sends the compressed file to the user's browser, which then downloads and unpacks it. Compression reduces the size of a file by up to 90% so that pages load faster and require less data usage. This, in turn, equates to a better user experience and higher search engine rankings.
How to fix the issue when highlighted
GZIP compression is enabled differently depending on which type of server you're using. One of the most common solutions is to add some code to the .htaccess file on your web server. You can find boilerplate GZIP configurations for popular servers here in GitHub.
Once enabled, GZIP compression will be detected during the next crawl and you will see your SEO Score improve accordingly.
For further details on the issues found click on the Google PageSpeed link on the GZIP Compression page.
Why do Siteimprove sometimes flag this issue when other services do not?
Siteimprove's GZIP Compression checks use data from Google PageSpeed (Lighthouse). If PageSpeed is finding resources on your site that are not compressed, then Siteimprove will report this as an issue.
When you fix the issue that PageSpeed is showing and re-crawl your site, you should no longer see this being reported in our SEO module.
You may get warnings from Google PageSpeed regarding externally hosted files that you do not have control over. In these cases, all you can do is to optimize the resources that you do have control over.
In some cases, Google does not flag an issue with text compression even though there are still files that can be compressed. They state, "If the original size of a response is less than 1.4KB, or if the potential compression savings is less than 10% of the original size, then Lighthouse does not flag that response in the results." In other words, a site can "pass" Google's text compression test threshold, but we still flag it as an issue. In this can you can still locate the issue by examining the text compression results shown in Google PageSpeed.
You can learn more about the PageSpeed tool and analyze a specific page on the Google PageSpeed Insights page.