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In Port Authority there are multiple places where information about buses is available.
There are information booths at the entrance to the terminal in both buildings.
There are electronic screens where you can look up information. They are now everywhere.
But if you're looking for Greyhound bus lines (not Peter Pan, not Pine Hill, or any other) you need to go to the 42nd Street building on 8th avenue and Greyhound is located downstairs. in the bas*m*nt. Once you get there there is Greyhound personnel and screens available for the information about where a particular bus is departing from.
You may be able to find more info on Port Authority of New York/New Jersey site.