A G5 would have to run the table and collect a noteworthy P5 scalp along the way; not a playoff contender necessarily, but it can't be Kansas, Wake Forest or Rutgers as your P5 win. Maybe a fallen giant like Tennessee or Nebraska would be best path. Taking down a big brand looks good, and both would be winnable games.
More often than not a team that pulls off a monster upset in September will fail to run the table in their league; AKA "pulling a Toledo."