Skill is relative. This is a long-term strategy game with some tactical elements. You have to be good at both. Planning your roster long term and playing the short-term battles/events/TW/etc.
While there are lots of metrics that could be used, doesn't mean they they are all useful for matchmaking. The problem is that pretty quickly you will run out of even matches. GP is the best constant. After that there are only so many people that the Arena top 200 or bottom 2000 people can fight against. Also, it is easy for a rank 2000 person to have the exact same roster and GP as a rank 200 person. They just don't know or care or have the ability to play at the same level. It is also possible for a 1.7M GP player to be ranked in top 10 with people that have 3M + GP. For the same reason you will probably get people of mismatched GP against each other.
Just because you are good enough to have a full g8 Phoenix team ready for the Thrawn event does not mean you can get Thrawn. Some people get him with g8, some have to have g10 characters to do it.
I'm guessing they dont only use GP as the sole criteria. I'm also guessing they will never tell us the exact formulae because min-maxers will job the system. I'm also sure they will contine to tweak it.
Just my opinion.