I have completed my graduation in 2017 and I am preparing for cat 2018 so will the 2 year gap affect in my selection in top b schools

No this 2 year gap doesn't effect much in your selection process. On course during your interview round they might ask you what you were doing these two years. If you are able to convince them with a logical answer that justify your actions then there won't be any problem at all.

With your decent academic records and a good performance in CAT 2018 you will definitely able to crack a top institute like IIMs.

As you have enough time to prepare then I will recommend you to make sure you clear all your basics and make them strong. For good performance strong basics are essentially required. Along with this you can also start attempting lots of online mock test which will help you in overcoming your mistakes and come up with a strategy to attempt your paper which will definitely help you in improving your marks and percentile.

All.the best for your preparations!!