How does GAC matchmaking work?

I took a little under a year off from the game and was wondering if someone could explain to me (or point me towards a post about) how GAC matchmaking works. I've been matched with people of very different GP's than my own (I'm about 3.4m, so in the middle of the division), and with way more relic characters than me. I've heard that matchmaking takes into account GP of your top toons and relic characters but it seems to be inconsistent about this. Thanks!