Unfortunately it is solely based on skill level for previous GAC wins/losses and the issues arise that what you are facing here is the evolution of how your opponents have fared (or not even tried).
So what you find is that some players hate GAC, join anyway because there is still a reward of some kind, then players think why not have a go. The GAC model will have lots of players staying around the same levels. High GP monsters may be dropping down to either tactically win and get promoted or just inactivity/no motivation. Then you will get highly involved lesser GPs push up, so its not GP that is the matchmaker here, its the history of wins/losses that will decide your skill rating, your opponents skill rating and the GAC bracket where the matchmaking occurs.
I never see anyone complain they fight lesser GPs....forget GP and realise that this is the game model. You have just been unlucky to get this matchup but everyone is in the same boat.