If you ask as per the riddle,
the centre of gravity is the letter "v".
If you ask scientifically, then you first need to understand that Gravity is not a force, but an effect of curving of Space -Time. The effect is caused by distortion of space-time by the presence of a heavy mass (like stars and planets) on it. In that sense, the center of gravity is the centre of mass of the object distorting space-time.