The issue isn't in the matchmaking. Conceptually, matching all players with a near-identical rating is the correct move; players that focused on this game mode exclusively should win more often than not, and players who focused on other game modes should lose most of the time. Everyone else will fall somewhere in-between.
The real problems are a) the calculation for GP is arbitrary and not really a good data point for comparison, b) the PvP modes of the game are structured in a way where whatever team goes first wins 80+% of the time, and c) there are too many teams with protection regeneration or resetting buffs (Saviour, Zombie, Bastila lead) that prevent attrition from being a viable strategy.