TW matchmaking is based on active GP and effective GP as stated by devs.
Active being the gp of the members who signed up. Effective being the GP - all units under 6000 GP.
These are both very good tools for matchmaking. In order to deal with having really strong guilds with few players dominating full weak guilds, I have always thought adding an "average GP per participant" as another metric would handle it nicely. To be honest they could replace effective GP with it. Just add one more step where it divides the effective by the number of participants. Though I'd rather they just add it as a new criterium.