In the 'help' tab of the ui-gauge there is a link 'Angular filters' that previously took you to a website with a tutorial all about the built in angularjs filters but the website no longer exists and you are taken to https://www.digitalocean.com/community.
A PR has been created to change the link to point to https://docs.angularjs.org/api/ng/filter
You can always do a google search using all about the built in angularjs filters to find other sites with explainations of how to use the Angular filters.
