You can run subs with their own master on the console, or you can run a summed L+R out, or you can run a simple stereo L/R setup similar to many analog consoles. What did you replace with the GLD-80, are you plugging into an existing system? You may want to keep it similar to the original setup initially until you get used to the GLD-80 and explore other options.
Personally for me I like the separate MONO + L + R, because it allows you to route only signals to the sub that you want like an Aux fed sub setup. You can then use a DCA to control your L/R & sub levels on a single fader. The GLD is very flexible and allows for many setup options.