The default ad lists are stored locally in
/etc/.pihole/, so you can just delete your existing list and then copy the default one back into place.
cp /etc/.pihole/adlists.default /etc/pihole/adlists.list
Then, update the gravity ad list using
pihole -g or updating the ad lists via the Web interface.
Another alternative is to remove the
/etc/pihole/adlists.list and then run
pihole -r, which will also accomplish the same thing by repairing your entire installation.
Note: Depending on what version you are on, you may have a
/etc/pihole/adlists.default, which is being deprecated for