Pella Skylight - Sash Groove

I recently installed a Pella fixed skylight and I am about to put up the drywall for the ceiling. I am curious about how I should finish the work between the skylight sash and the ceiling drywall. Do I use drywall or wood? The skylight sash is about 1-1/4" thick with a groove around the perimeter facing into the room that is about 1/2" wide. Is this a standard groove configuration? If so, will I find mating lumber at the home improvement stores, or do I need to do some fancy router work?
TIA for any replies, JP

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