Answer pulled from here: https://askubuntu.com/questions/470040/how-to-customize-or-debug-ad-blocking-on-gnome-web-epiphany-browser
Gnome-web blocks ads on the basis of the URI of the element. The blocking is
done by the UriTester componant (source code). Like Adblock plus, the tester
loads a list of patterns from a list, and determine if a URI is an ad if it
matches one of the pattern.
The file $HOME_DIR/.config/epiphany/adblock/filters.list contains the
locations of the lists to load (one on each line). If this file doesn't
exists (e.g. first time you run epiphany), it defaults to "https://easylist-
downloads.adblockplus.org/easylist.txt". Gnome-web then download (and
periodically refresh) the pattern lists, and store them in the same
directory (with file name like bedc7ed868397c3dcaf6bc40ed9bbf70)
So, in case of troubles, check that you have filters.list, with one line
pointing to a valid adblock+ filter, and one file with a file name of 32 hex
chars, containing valid blocking pattern. If the file filters.list exists,
but is empty, ads won't be filtered at all.
It is also possible to customize the list used by changing the url in
filter.list (for instance replacing the default list with a localised list
like https://easylist-downloads.adblockplus.org/liste_fr+easylist.txt).