Gravitational Field

The space around a body within which its gravitational force of attraction is experienced, is called its gravitational field.

Gravitational field strength or gravitational field intensity

The gravitational field intensity at a point is defined as the force experienced by a unit mass placed at that point.

Intensity of the field experienced by a test massLet M be the mass of a body around which a gravitational field exists. Let m be a test mass placed in this field. The test mass is so chosen that it does not influence the gravitational field. The arrow over indicates it is a vector quantity, whereas mass is a scalar quantity.
A force  field is the force experienced by the unit mass m. The intensity  of the field is the force experienced by the unit mass.

If the test mass m were free to move, it would move with an acceleration towards M. 
 The gravitational field intensity is defined as the free acceleration experienced by a test mass at that point.
The units in the

• C.G.S. system are dyne gm^-1 or cms^-2

• SI system are Nkg^-1 or ms^-2