GATE has subjects like OS, Data structures, Computer Architecture, Digital Logic design, Compilers, DBMS, Theory of Computation,  AND Maths is a must. It depends upon the person who sets the questions as equal weitage has to be given , But as far as i view the previous question papers i can clearly say these are very important- Data structures, OS, Networks, Digital Logic design and Maths

Also remember u have 15 marks in aptitude