Can gravitational force act as an impulsive force in any case? Can gravitational force be termed as impulsive in any case? I' m thinking about it but couldn't get any answer.
When two heavenly objects collide then the force exerted on each of them during the time of impact is due to gravity, hence can it be termed as impulsive?
 A: 
Can gravitational force be termed as impulsive in any case?

Yes, but not in the circumstance you described.

When two heavenly objects collide then the force exerted on each of them during the time of impact is due to gravity, hence can it be termed as impulsive?

While gravity may have drawn the objects together, it plays a trivial role during the actual collision. A number of other forces that completely overwhelm gravity are involved in a collision of heavenly bodies other than gravity.
Consider the Chicxulub impactor, a roughly 2 teraton object that collided with the Earth at about 20 km/s. The collision stopped that impactor in about one tenth of a second. The force needed to stop a 2 teraton object moving at 20 km/s is about 4×1020 newtons, directed against the velocity vector. A force that large exerted over that a short of a period of time qualifies as "impulsive".
However, that force wasn't gravitation. The gravitational force at the time of impact was about 2×1016 newtons, directed downward. That's about four orders of magnitude smaller than the impact force. Gravitation was negligible during the immediate contact.
It's important to note that there is no such thing as an impulsive force, a force that instantaneously change momentum. People model forces as impulsive because doing so can make analysis much simpler. Modeling a force as impulsive is approximately valid when that dominates over all other forces and when that dominant force is only applied for a short period of time. Gravitation is a small force whose effects only becomes significant when applied over long periods of time.

That said, there is a place where gravitation can be treated as impulsive. Imagine a satellite moving toward a planet. Suppose the satellite comes close to but does not impact the planet, and then moves on. This is the basis for a gravity assist. Most of NASA's deep space missions take advantage of gravity assists. During the flyby, the gravitational force exerted by the planet on the satellite is several orders of magnitude large than the gravitational force exerted by the Sun on the satellite. The period of time over which the planet had a significant effect on the satellite is short compared to the orbital period of the planet. The flyby thus can be modeled to some extent as impulsive.
