Asterisk Dial plan for Multiple carrier

I am in need of an Asterisk dial plan for outgoing calls. The primary purpose of this plan is to distribute calls equally among multiple GSM gateways, of which I have between 4-7. Specific features required for this plan include time-based routing and call routing. It is important that calls are distributed equally among the available gateways as allowed ports. Ideal skills and experience for this project include knowledge of Asterisk, dial plan creation, and experience with multiple carrier systems.