It's a lot of little things that if you could see them in person side x side, would be obvious. Lighter gauge sheet metal, fewer bends/folds for stiffening, a little less (but still minimum) copper in the current carrying parts*, the handle is easier to bend, stuff like that. And apparently Dq D are ones who don't put the cover interlocks on any of the GD switches; if it were Siemens the 60A switch would have that.
*Sq. D (and others) uses what's called a "live hinge" type of mechanism that is basically a bar with a pin in one end that acts as the pivot point and when closed, wedges between two bars forming a slot on the other side. Current must flow though the moving parts, so the hinge parts must be kept tight with spring pressure. In the GD switch, everything about this arrangement is lighter, less tight etc. as time goes on and the switch sees more operations, that live hinge on the GD switch will loosen up and start getting hot. On the HD switch, the parts are stronger, stiffer and the springs are better. It's the difference between barely passing test requirements and exceeding them.
All that said, if it's a resi installation that will likely not see 10 operations per year, you may never notice. In other words HD would be overkill.
