General aptitude is a vast area but for GATE they ask only from a small section which is clearly mentioned in GATE syllabus. Many people won't prepare for these but it is advisable to just read the sections mentioned in GATE syllabus from a recommended book for aptitude such as Aggarwal.
Discrete Mathematics is an important area in GATE CSE. This you should definitely read from one of the recommended books such as Kenneth H. Rosen.
For more books: http://gatecse.in/gatecse/index.php?title=Best_books_for_CSE