The only so-called "legitimate" way of entering the Void is by glitching, though the "Grey Zone" will most likely inhibit you from reaching your final destination.
If you somehow DO make it into the Void, one wrong step may leave you falling into it. And as you may have guessed, falling into the Void causes a player to fall under the map, which eventually results in death.
There are now two "shades" of the Void, the darker shade and the lighter shade. Staying in the lighter side is safer, as the screen remains relatively bright and you won't die. Venturing too far past that, however, leaves you in the darker side, which will automatically Nexown you if you remain there for too long.
Most maps (such as Gray Hammer) may seem very large, but approximately 35% of the map is playable (accessible). The other 65% consists of the area AROUND the so-called "map," which takes the form of an incredibly large field-like stadium.
Once you reach the "End" of the Void, you cannot move any farther or glitch past that area (because you've already reached the limits of the map coding). Attempting to do so may crash your game.