Grinders seem to be native to southern New England. A gyro is different - it's wrapped in a flatbread. A hero, pronounced the same, has the long bread.
It probably isn't true anymore, but I think the names used to correlate with regional variations in ingredients. When I was little, I liked subs and hoagies. My family moved to Mississippi and I encountered po' boys, which are the New Orleans species. I never liked them much because they were usually dripping mayo or some other gooey sauce. Every little grocery and bar in New Orleans has outstanding red beans & rice and muffelettas, so po' boys are easily avoided.
I moved to CT as a high school senior and discovered grinders, which are the kings of sandwiches. At face value, grinders don't seem very different from the other species. Most come with the standard lettuce, onions, and tomatoes. Maybe the difference is they also get provolone cheese, roasted peppers, and vinegar & oil. Eggplant, meatball, and sausage grinders come with cheese and roasted peppers and sauce. Egg & pepper grinders come with nothing at all.
But some places sell grinders that are really subs. An easy way to identify such a fraud is they'll ask if you want mayo, mustard, or vinegar & oil. If that happens, head for the door because a real grinder place would never ask. It would be as if a bartender offered whipped cream or sprinkles when you ask for a beer. Run. The only questions a grinder place should ask after you order are whether you want it hot or cold and here or to go?