I have a GMG DB and a FEC-100 so I know pellet cooking fairly well.
I'd place the bird right in the middle of the grill since the fire-pot is underneath it. I'm assuming you've left it in stock configuration in that the heat deflector and drip pan are in place. That will provide a good diffusion of heat and keep the cooking pretty even. If you want the drippings you may need to use a rack with a pan etc.
I'd cook at ~325-350 on the GMG until the internal is ~165 to 170 depending on where you are measuring.
The beauty of the pellet cookers, and the GMG is no different, is that you will get an oven like regulation of temperature so think very similar in terms to cooking it in an oven.
One trick I've used in the past if I want some extra smoke is to prepare an aluminum foil packet with a couple small holes in it and put wood chips in it. Place this ontop of the heat shield (not the drip pan) and you will get some extra smoke out of the cooker.
Hope it goes well. Post some pr0n!