It was my understanding that the P4 have an auto-bid to the CFP as does the highest ranked G5 team, but of those 5 champions, only the top 4 get a bye for the first game. So if the G5 team champion is ranked higher than the lowest ranked P4 champion, then they snag they bye for the first round.

I don't think that a second highly ranked G5 champion could ever displace any of the P4 conferences in terms of their auto-bids to the CFP, just the bye position, but I could be wrong about that. Just the way I understood it from the ESPN commentary discussed in this thread.