I have both bows, and while I am just getting into the recurve game, here are my uneducated observations:
Finish: I have the carbon Panther and mostly because of the milling techniques, it has a lot of sharp corners and edges that are starting to silver. My GM even though I bought it used, shows little wear.
Fit: The limb to riser fit is similar and snug for both. Martin supplies a set of limb pads to dampen vibration, you have to spend about a dollar to make your own for the Hoyt.
Design: The GM and Panther look alike, but there are geometry difference. Here is where my lack of recurve experience may get me into trouble, but I don't find one to be easier or more forgiving to shoot than the other. One is slightly reflexed (Panther) and one is slightly deflexed (GM), so the consensus is that the deflexed GM is more forgiving and the Panther is "twitchy." Again, I can't feel the difference.
Limbs: Better limbs, by virtue of the fact that any ILF limb can be made to fit, will be available for the GM; from the mild to the wild! Having said that, there are some limb options for the Martin, since there are several other bows that use the same limb pocket dimensions (Samick, OMP, Ragim, etc...) but the quality hits a ceiling quite a bit lower that what is available for the GM.
Other points to note: I do like the vibration dampening on the Martin and it seems to work. After finding the sweet spot in the brace height and tying on cat whiskers, the Martin is very quiet of of the supplied plastic center rest. My GM took a little more time to get as quiet. The grip on the Martin is not terrible, but I am shooting it often enough that I am wearing a hole in the rubber that wraps around the front of the riser. The leather on the back of the grip wrinkled up in a week and was removed. Hoyt has a wooden grip, and custom grips are available. If you plan to shoot off of the shelf at some point, I would get the GM. The Martin has a rubber covered shelf and is cut like a compound bow. Perfect for an elevated rest, but will require a lot of build up for the strike plate. The GM riser is less cut beyond center and has a radius cut shelf; easily set up for shooting of of the shelf.
Do I think that the Martin is garbage, next to the GM? Heck no. If you can find one used and had a tight budget, the Panther is a capable bow. It has it's limitations, but propelling arrows in an accurate manner is not one of them. Is the GM a perfect bow? No, but it definitely gives the shooter more options not available on the Panther.
I hope that helps.
cricman