I am doing my undergrad final year . For securing marks in the same , I could not prepare for CAT satisfactorily. Is a gap year advisable ?

yes you can drop a year and properly prepare for CAT. But if you have the passion to do it then only drop a year else it will be a proper wastage of your precious time. So all the best and move ahead in the journey of following your dreams.

Well,  if you are really determined to set your career in MBA field then do not silent your dreams. Go for it. But yes, do not let your year gap be a complete year gap. Apart from preparing for CAT, try doing some other part time job or part time course, like for instance there are certain universities which provide one year diploma programs in some languages or courses like HR, etc. You can also do Adwords certification of Google. Or some software based courses like SAS, SAP, etc. Also if possible you can look for some internship options in industries near your locality and ask them to give you certificate for the same. Try to make it little productive too so that you have an answer for your interview that you wanted to prepare yourself better that's you dropped for a year to do some courses pertaining to MBA.
All the  best!!