You have indicated that you have a high-conflict case, and you have not indicated that there is any basis upon which the judge might reconsider his decision to appoint a GAL, so at this point we have no way of commenting either on what you believe to be the grounds for removal or why a judge might find your argument to be compelling.
If you and your ex- do not want a parenting coordinator, the best way to demonstrate that one should not be appointed is to start working cooperatively on parenting time issues. The idea of the coordinator is to try to keep smaller disputes and conflicts out of court, and at least in theory the appointment should provide for faster resolution of conflicts. Having to pay the parenting coordinator's fees is sometimes also enough to inspire parents to stop arguing over trivialities. I don't know whether parenting coordinators in your county have the power to make decisions, as opposed to merely making recommendations.
My personal preference is to keep as few third parties in the case as possible. But if the parents cannot cooperate and keep bringing their case back to court, I understand why the judge starts looking to additional professionals -- referees, mediators, custody evaluators, GALs, parenting coordinators, and the like -- to keep his docket clear for the rest of his cases.

