Why do we have electromagnetic waves only in the wave lengths between 10^2 to 10^-14 metres respectively from radio to gamma waves.? Does any scientific reason exist for that? If it is not so, Does any other waves exist in the extremes of those both. Does gravitational waves belong to electromagnetic waves.,?
There is no physical principle that á priori restricts the range of possible wavelengths of electromagnetic radiation.
However, the higher the frequency the more energetic the radiation will be, so it will be harder to make that kind of radiation.
As for low frequency you can also have as long a wavelength as you wish. In fact, you can think of a static electric field as roughly the limit of the wavelength going to infinity.
Finally: No, gravitational waves are NOT electromagnetic radiation.
When it comes to gravitational waves, no we have not determined them to be EM waves. Although we can't completely rule it out. For example, there is a theory that reconciles EM force with weak nuclear, and it is called Electroweak. Electroweak interactions happen above 100 GeV, when EM and weak nuclear become the same force.
Edit: Maybe someone will develop a theory that unifies gravity and EM as the same force. Meanwhile, this page from CalTech might help answer your gravity wave question.