I need good books for csit in gate exam

I would suggest you to go through the below details -
Engineering Mathematics -Discrete Mathematics and Its Applications by Kenneth Rosen, Advanced Engineering Mathematics by Erwin Kreyszig.
Digital Logic - Digital Design by Morris Mano, Digital Electronics by S. Salivahanan & S. Arivazhagan.
Computer Organization and Architecture - Computer Architecture – A Quantitative Approach by Hennessy and Patterson, Computer Organization and Embedded systems by Carl Hamacher.
Programming and Data Structures - The C Programming language by Dennis Richie, Classic Data Structures by Debasis Samanta.
Algorithms - Introduction to Algorithms by Cormen, Data Structures & Algorithms by R.S. Salaria.
Theory of Computation - An Introduction to Formal Languages and Automata by Peter Linz, NPTEL lectures by Kamala Krithivasan.
Compiler Design - Compilers: Principles, Techniques and Tools by Aho & Ullman, Compiler Design in C by Allen I Holub.
Operating System - Operating System Concepts by Galvin, Operating Systems by Stallings

Databases - Database System Concepts by Henry. F. Korth, Normalization and indexing by Shamkant B. Navathe.
Computer Networks - Data Communications and Networking by Kurose & Ross and Behrouz A. Forouzan, Computer Networks by Peterson & Davie.

Hope this helps!!!