Starting with version 18.4.0 GitLab needs another PG extension: amchek.
To list the current PG extensions run as the PG admin user:
psql -d <your GitLab DB> -c "\dx"
To create the extensions if they are missing do:
psql -d template1 -c "CREATE EXTENSION IF NOT EXISTS amcheck;"
psql -d template1 -c "CREATE EXTENSION IF NOT EXISTS pg_trgm;"
psql -d template1 -c "CREATE EXTENSION IF NOT EXISTS btree_gist;"
psql -d template1 -c "CREATE EXTENSION IF NOT EXISTS plpgsql;"