Just to set the record straight, the many variables involved are:

If the head is the moving object.

The starting velocity of the head.
The ending velocity of the head.
The combined deflection/distortion of the head and object hit.
This can include padding of a hat, dashboard, etc. or flexing of a wall or
movement of the object.
Any angle of incidence (The angle the head was traveling compared to the
surface of the object.)
The time of head contact with the object surface until reaching the ending
velocity. Knowing the combined deflection/distortion makes this value
not critical.
Any rebound/bounce of the head. This is a difficult value to calculate and
add to the formula

If the head starts stationary and the object is moving.

The velocity of the object before contact with the head.
The velocity of the object after the contact.
The deflection/distortion of the object.
The deflection/movement of the head and angle of movement.(related to
the angle of incidence)
The weight of the object.
The weight of the head. (5000 grams is commonly used)
The weight of the object and weight of the head can be used to
determine the movement of the head. The deflection of the object is
still needed.
Any stabilizing forces holding the head in place. This is a value that is hard
to determine and a complex to add into the formula. It is not likely to
make a big difference in the end result.
The angle of incidence.

There may be some other variables but they would not change the value by much in most situations, such as, did the head start to rotate ?

Most of these values will be estimations or assumptions. But, by using highs and lows, a range of G forces can be calculated.