There already exists command apt-cache dumpavail which will list all available packages from all enabled repositories. Behind the scenes it actually reads from files stored in /var/lib/apt/lists/ directory (I've done strace of the command, and that's what the output shows) . My guess would be that Gnome Software parses those very same files and organizes those into categories.

Problem is that the actual data has lines that start with Package: for package names and Section: to which they belong, but Section: lines are not organized exactly the same as in Gnome Software. However, with a little bit of command line magic, we can come close to something like that. What I propose is a function

filter_sections()

{

  apt-cache dumpavail | 

  awk -v SEARCH="$@" '/^Package:/{ PKG=$0  }

    /Section:/ && $0~SEARCH {printf PKG" "$0"

"}'

}

With that function we can list all packages by sections, for instance:

$ filter_sections web | head                                                   

Package: apache2 Section: web

Package: awstats Section: web

Package: curl Section: web

Package: heat-api Section: web

Package: heat-api-cfn Section: web

Package: heat-api-cloudwatch Section: web

Package: heat-common Section: web

Package: heat-engine Section: web

Package: javascript-common Section: web

Package: libapache2-mod-apparmor Section: web

The code itself is fairly simple: we pass on the output of apt-cache dumpavail to awk which stores every package name into a varialbe , and if the Section: line also contains a string that we are matching, we will print both package name and the section.

As for listing the sections themselves, it's fairly easy as well

apt-cache dumpavail | awk '/Section:/' | sort | uniq 

What is also nice about this approach is that some of the Sections mention which repository the package belongs to, for example universe/python or multiverse/web . The function , however, will search for all and of them , but if so desired we can always filter with awk even more