Have a Traeger, but only really because it was a Costco special. We love it. I might have gone GMG if it weren't for Costco. Both are good choices.
However, as a caution, if you use an external meat thermometer with the Traeger, wrap the wires where it enters the grill in some foil for protection. I went through several probes until I read that trick. Don't know if GMG has that heavy of a lid, but thought I'd throw that out there to save a little heartache.
You'll love grilling with pellets. Anything you can bake, you can also cook with pellets using the same recipes. One of our best was a whole turkey. Happy eating!