Option 1: Push force - This refers to the force exerted by an object when it is pushed by another object. It requires direct contact between the two objects. Therefore, push force is a type of contact force.

Option 2: Gravitational force - This is the force of attraction between two objects with mass. Unlike contact forces, gravitational force does not require direct contact between the objects. This force acts even when there is no physical contact between the objects. Therefore, gravitational force is not a contact force.

Option 3: Frictional force - Frictional force is the force that opposes the motion of an object when it comes into contact with another object. It only occurs when there is contact between two surfaces. Hence, frictional force is a type of contact force.

Option 4: Strain force - The term "strain force" is not commonly recognized in the context of forces. It is possible that this option is referring to a different force that was not explained in the question. Without further information, it is unclear if strain force is a contact force or not.

In conclusion, the correct answer is option 2: Gravitational force. It is not a contact force as it does not require direct physical contact between objects.