I wish to the CAT exam in 2018 and would do my graduation in 2018 April. so will that be considered as a gap because the exam is in November

It will be a gap but will have no impact on your shortlisting for interviews. For an interview call mostly institutes take the qualifying exam's percentile while in top IIMs and some others they do a profile based shortlisting considering diversity, academics etc.

So you will get an interview call based on your score and then in the interview you 'might' be asked the question about the gap. It is totally upto you how you convince them about the gap and usually it is not very hard to convince them.

So focus on your preparation and try to get the best percentile. All the best!