Does it exist?
Supposedly, it can be found about 4 centimeters inside the rectum, toward the belly side.
The same people who say it exists also say that the best way to stimulate it is thus:
The man lies on his back, legs apart and bent at 90 degrees. His partner inserts a finger (palm up) and stimulates the G-spot by 'come here' motions with the finger.
If done correctly, the man should come here.