Our guild is probably around 55% wins at 450 mil GP. It is encouraged to only join if you can actually attack, so most of the time around 5 people sit a TW out.
The match ups always seem to be 'fair', insofar as the algorithms seems to hold us around a 50% win rate, so it seems fine for us even if some players 'sandbag' by not joining.
What could be implemented to make it fairer in huge GP gap matches is a handicap for the bigger guild. Maybe their defense has more slots to fill, while at same time maxxing out the points at the defense max of the weaker guild or something, so the members have less teams for offense.