Grand Arena Championships – GP Divisions [MEGA]

I found this from dev post.

If the GP difference across players is great enough, the matchmaking will also default to the lowest-number of defenses required by division between the two players (Note: only the very best-performing low-GP players are likely to encounter this, however). Players who are good enough to end up getting matched with higher-GP players will be able to do so on a more level playing field.